Nolan is now among the most common surnames in Ireland. It is an anglicised form of O Nuallain, from a diminutive of nuall, meaning 'famous' or 'noble'. The family are strongly linked with the area of the modern Co Carlow, where in pre-Norman times they held power in the barony of Forth, whence their ancient title of 'Princes of Foharta'. Their power was greatly diminished after the arrival of the Normans, but the surname is still strongly linked with the area. The prevalence of the surname in the modern counties of Mayo and Galway is explained by the migration of a branch of the family to that area in the sixteenth century; they obtained large tracts of land, and their descendants are many.
The most famous modern bearer of the surname was Brian O'Nolan (1911-1966), better known under his two pen-names of Flann O'Brien and Myles na Gopaleen, whose genius for comic invention has only been fully appreciated since his death.


O'Brien is in Irish O Briain, from the personal name Brian. The meaning of this is problematic. It may come from bran, meaning 'raven', or, more likely, from Brion, a borrowing from the Celtic ancestor of Welsh which contains the element bre-, meaning 'hill' or 'high place'. By association, the name would then mean 'lofty' or 'eminent'. Whatever the initial meaning of the word, the historic origin of the surname containing it is clear. It simply denotes a descendant of Brian Boru, ('Brian of the Tributes'), High King of Ireland in 1002, and victor at the battle of Clontarf in 1014. He was a member of the relatively obscure Ui Toirdealbhaigh, part of the Dal gCais tribal grouping based in the Clare/Limerick area. Having secured control of the Dal gCais in 976, he defeated and killed the Eoghanacht king of Munster two years later, and proceeded to wage deadly war against the kingdoms of Connacht, Meath, Leinster and Breifne. Eventually he secured submission (and tributes) from all but the northern Ui Neill, the Leinstermen and the Vikings. His victory at Clontarf united all of Ireland, nominally at least, under a single leader, though Brian himself was slain. The first individual clearly to use O'Brien as a genuinely hereditary surname was Donogh Cairbre O'Brien, son of the king of Munster, Donal Mor. His descendants split into a number of branches, including the O'Briens of Aherlow, the O'Briens of Waterford, the O'Briens of Arra in north Tipperary, and the O'Briens of Limerick, where the surname is perpetuated in the name of the barony of Pubblebrien. Today the name is numerous and widespread throughout Ireland, with particular concentrations in these areas, as well as in the original homeland of Clare. The surname has been prominent in all sphere of Irish life. The novelist and dramatist Kate O Brien (1897-1954) suffered, like most Irish novelists of worth, at the hands of the censors in the early years of the Irish Free State. William Smith O'Brien (1803-1864) was one of the founders of the Young Ireland movement, and took a prominent part in the rising of 1848. His grandson Dermond O’Brien (1865-1945) was a leading portrait painter in Dublin for almost forty years.

O'Callaghan, along with its variants (O)Callagan, Callahan etc., comes from the Irish O'Ceallachain, from the personal name Ceallachan, a diminutive of ceallach. This was traditionally taken to mean 'frequenter of churches', but is now thought to be a much older word meaning 'bright-headed'. The personal name was much in favour among the Eoghanacht, the tribal grouping who controlled the kingship of Munster before the rise of Brian Boru of the Dal gCais, and it is from one of the Eoghanacht kings, Ceallachan (d.964), that the family trace their descent. Murchadg Ua Ceallachain, a grandson of this king who lived in the early eleventh century, was the first to transit the surname hereditarily. His nephew Carthach was the ancestor of the MacCarthys, and a bloody succession feud between the MacCarthys and the O'Callaghans continued well into the twelfth century, ending with the MacCarthys in the ascendant. By the end of the thirteenth century the O'Callaghans had taken decisive possession of that part of Co Cork which came to be known as Pobal Ui Cheallachain, O'Callaghans Country. This was a very large area on both sides of the river Blackwater west of the modern town of Mallow. Here their principal bases were the castles at Clonmeen and Dromaneen, and from them they retained virtually uninterrupted control for over four centuries, containing many of the earlier Gaelic customs. The most notorious of these was the creach or cattle-raid; one Donncha, chief of the family from 1537 until his undeservedly peaceful death in 1578, was reputed to have carried out two hundred raids in every county of Munster, evidently regarding the creach as a vital part of his cultural inheritance. In the great confiscation’s following the wars of the seventeenth century the family lost virtually everything. The ruling chief, Donncha O'Callaghan, and his extended family were transplanted to east Clare, where they obtained land in the barony of Tulla. The village of O'Callaghans Mills records their continued presence. Like so many others from the old Gaelic aristocracy, members of this Clare family emigrated to continental Europe. Cornelius o'Callaghan entered the army of Spain in 1717. In 1944 one of his descendants, Don Juan O'Callaghan of Tortosa, was recognised by the Genealogical Office as the senior descendant in the line of the last inaugurated chief, the Donncha who was transplanted to Clare.

O'Connell, along with Connell, generally comes from the Irish O Conaill, 'descendant of Conall', a very popular personal name probably derived from con, 'hound' and gal, 'valour'. Because of the widespread popularity of the personal name at its root, O'Connell arose separately as a surname in Connacht, Ulster and Munster. However, by far the most prominent and numerous of these were the O'Connells of Munster, where the family were originally lords of the barony of Magunihy in east Kerry. Driven from this area by the O'Donoghues, they moved south and the centre of their power shifted to Ballycarbery, also in Co Kerry. Today a large majority of the O'Connells in Ireland are still to be found in Co Kerry, as well as in adjoining Co Cork. This family produced the most famous bearer of the name, Daniel O'Connell (1775-1847), known as 'the Liberator' because he won Catholics the right to vote; for almost thirty years he was the undisputed leader of Catholic Ireland. In Ulster, especially in counties Antrim, Tyrone and Down, many Connells and MacConnells are of Scottish stock, their names derived from a phonetic transliteration of Mac Dhomhnaill, since the 'Dh-' is not pronounced. This family were a branch of the great Clan Donald.

O'Connor, with its variants Connor, Conner, Connors etc., comes from the Irish Conchobhair, from the personal name Conchobhar, prehaps meaning 'lover of hounds' or 'wolf-lover'. This was one of the most favoured of early Irish names, and gave rise to the surname in at least five distinct areas, in Connacht (O'Conor Don), in Offaly (O'Conor Faly), in north Clare (O'Conor of Corcomroe), in Keenaght in Co Derry, and in Kerry (O'Connor Kerry). The Offaly family take their name from Conchobhar (d.979), who claimed descent from Cathaoir Mor, a second-century king of Ireland. They remained powerful in their original homeland until the sixteenth century, when they were dispossessed of their lands. The O'Connor Kerry were chiefs of a large territory in north Kerry, displaced further northwards by the Norman invasion to the Limerick borders, where they retained much of their power down to the seventeenth century. Today, the descendants of these O'Connors are far and away the most numerous, with the majority of all the many O'Connors in Ireland concentrated in the Kerry/Limerick/Cork area. However, the most famous of all the O'Connor families is that which arose in Connacht. The ancestor from whom they take surname was Conchobhar, King of Connacht (d.971), and direct ancestor of the last two High Kings of Ireland, Turlough O'Connor and Roderick O'Connor, who ruled through the twelfth century. Unlike the vast majority of the rest of the old Gaelic aristocracy, the O'Conors of Connacht managed to retain a large measure of their property and influence through all the calamities from the seventeenth century on. The line of descent from the last Chief of the Name is also intact; the current 'O Conor Don', recognised as such by the Chief Herald of Ireland, is Denis O Conor. The family seat remains in the ancestral homeland, in Castlerea, Co Roscommon.

O'Donnell comes from the Irish O Domhnaill, 'descendant of Domhnall', a name meaning 'world-mighty'. Given the popularity of this name, it is not surprising that the surname containing it arose simultaneously in a number of areas, among them west Clare and east Galway, where they were part of the Ui Maine, the sept grouping under the control of the O'Kellys. The most famous O'Donnells, however, are undoubtedly those based in Donegal. Like many northern families, the O'Donnells of Tir Chonaill were part of the great Ui Neill tribal grouping, claiming common descent from Niall of the Nine Hostages, the fifth-century monarch who is reputed to have kidnapped St Patrick to Ireland. They were not prominent in early times, inhabiting a relatively small territory around Kilmacrenan. From the late Middle Ages, however, their power and influence grew steadily until, by the fourteenth century, they were undisputed lords of Tir Chonaill, roughly identical to modern Co Donegal. Their dynasty continued for more than three centuries, culminating with their involvement in the Nine Years War, in which Red Hugh O'Donnell (1571-1602) and his brother Rory, First Earl of Tyrconnell (1575-1608), played a famous part, almost inflicting a decisive reverse on the progress of English rule. The defeat suffered by the alliance of the remaining pre-eminent Gaelic families was the beginning of the end for the old order in Ireland. Rory O'Connell was one of those who took part in the 'Flight of the Earls', the departure from Lough Swilly in Donegal in 1607 of the most powerful remaining Irish leaders. Unlike many others among the old Irish aristocracy, however, the line of their descent remains unbroken. The last duly inaugurated chief was Niall Garbh ('Rough'), and a direct line of succession from his younger brother Hugh Buidh ('Yellow') continues down to the present. The present 'O Donnell of Tirconnell', recognised as such by the Chief Herald of Ireland, is Aodh O Donnell, now living in Dublin.

O'Donoghue, with its variants Donohue, Donahoe, Donohoe etc., comes from the Irish O Donnchadha, which derives from the popular personal name Donncha from donn, meaning 'brown'. The surname would thus mean literally 'descendant of the brown-haired ( of brown-complexioned ) man'. The popularity of the personal name meant that the surname arose independently in a number of places, including Galway, Roscommon, Cork, Tipperary and Cavan. The anglicised versions vary slightly, with 'Donohoe' more common in Galway and Cavan. The most important of these families historically speaking were the O'Donoghue of Desmond, or south Munster. These were part of the Eoghanacht peoples, dominant throughout the south of the country until the rise of the Dal gCais under Brian Boru, and shared their ancestry with the O'Mahonys. Like the O'Mahonys, the Desmond O'Donoghues saw their power greatly diminished by the steady rise of the MacCarthys.Ultimately they were completely displaced from their original homeland in west Cork, and settled in southwest Kerry. Here they split into major groupings, the O'Donoghue Mor, based around Lough Leane near Killarney, and O'Donoghue of the Glen, based in Glenflesk. O'Donoghue Mor shared the fate of the majority of the old Gaelic aristocracy, dispossession and poverty, but the O'Donoghue of the Glen managed to retain both the family property and the unbroken succession to the title through all the vicissitudes of the last four centuries. Geoffrey O Donoghue is the current bearer of the title O'Donoghue of the Glens, recognised as Chief of his Name by the Chief Herald of Ireland.
Among the many bearers of the name are Juan O'Donoju (1751-1821), the last Spanish ruler of Mexico descendant of an O'Donoghue emigrant to Spain; John O'Donoghue (1900-1964), a novelist who wrote movingly and simply about his experience of rural Ireland, and David James O'Donoghue (1866-1917) poet, librarian and man of letters.

O'Donovan comes from the Irish O Donndubhain, from donn, 'brown' and dubh, 'black' or 'dark', the surname thus meaning 'descendant of the dark brown (-haired/complexioned) man'. The original Donnduban from whom the surname derives was king of Ui Chairpre in O'Grady, in 1309, and has remained the principal seat of the family down to the present day. Unlike so many other of the native aristocracy, the O'Gradys sided with the English in the sixteenth century, and intermarried with a number of powerful English families, thus retaining their influence and possessions through all the vicissitudes of the seventeenth and eighteenth centuries. Two of those marriages, that of Darby O'Grady to Faith, daughter of Sir Thomas Standish of Lancaster in 1633, and of John O'Grady to Mary Elizabeth de Courcy, daughter of Baron Kinsale, are reflected to this day in the personal names in use in the family; the present O'Grady of Kilballyowen, popularly 'the O'Grady', and recognised as such by the Chief Herald of Ireland, is Gerald de Courcy O Grady. The most prominent historical bearers of the name were Standish Hayes O'Grady (1832-1915) and his cousin Standish James O'Grady (1846-1928). Both were deeply involved in the nineteenth-century revival of interest in the Gaelic past of Ireland, the former as a renowned scholar and student of early Irish history and society, the latter as a popular novelist who based his stories on Irish legends and history.

O'Grady, along with Grady, comes from the Irish O Gradaigh, from gradach, meaning 'noble'. The surname originated in Co Clare, where the O Gradaigh were part of the Dal gCais tribal grouping who claimed descent from Cas, a son of Oiloll Ollum, the semi-legendary third-century king of Munster. They thus shared common ancestry with the pre-eminent family of the Dal gCais, the O'Briens, and took a prominent part in the O'Briens' struggle against the rival Eoghanacht MacCarthys, descended from Eoghan, another son of Oiloll Ollum. Although Clare was their homeland, from a very early date the family had strong associations with Co Limerick, in particular the area around Kilballyowen. This was acquired by the then head of the family, Hugh Donal, born c.943; the individual on whom he based his name was Niall Glun Dubh ('Black Knee'), High King of Ireland who died in 919. In the fourteenth century a branch of the Tir Eoghain O'Neills migrated eastwards and, under the leadership of Aodh Buidhe ('Yellow Hugh'), wrested large areas of Antrim and Down from Norman control. The territory at the centre of their power, Clandeboy, took its name from them (Clann Aodha Buidhe), and they in turn became known as the Clandeboy O'Neills. Their principal castle was at Edenduffcarrig, northwest of Antrim town, still occupied by an O'Neill. The present titular head of this branch of the family is Hugo O'Neill, 'O Neill of Clandeboy', a Portuguese businessman descended from Muircheartach, chief of the family from 1548 to 1552. The descent of the original Tyrone family has also continued unbroken, down to the present holder of the title of O Neill Mor, Don Carlos O'Neill of Seville, who also holds the Spanish titles of Marques de la Granja, Marques del Norte and Conde de Banajir. He is descended, through the O'Neills of the Fews in Co Armagh, from Aodh, second son of Eoghan, inaugurated as chief of the name in 1432.

O'Hara is a phonetic anglicisation of O hEaghra. The family clan descent from Eoghra, lord of Luighne (the modern Leyney) in Co Sligo, who died in 976 and who was himself, in the traditional genealogies, of the family of Oiloll Ollum, king of Munster. The O'Haras remain strongly associated with Co Sligo, where they were chiefs in two areas, O hEaghra Buidhe ('fair') around Collooney, and O hEaghra Riabhach ('grey') at Ballyharry, more properly 'Ballyhara'. In the fourteenth century a branch of the family migrated north to the Glens of Antrim and established themselves in the area around the modern town of Ballymena. There they intermarried with powerful local families and acquired great prominence themselves. Apart from Dublin, Sligo and Antrim are still the two regions where the surname is most concentrated.

O'Keeffe, and Keeffe, are the anglicised versions of the Irish O Caoimh, from caomh, meaning 'kind' or 'gentle'. The original Caomh from whom the family descend lived in the early eleventh century, and was a descendant of Art, King of Munster from 742 to 762. Originally the territory of the family lay along the banks of the Blackwater river in Co Cork, but the arrival of the Normans displaced them, like so many others, and they moved west into the barony of Duhallow, where their territory became known, and is still known, as Pobal O'Keeffe. The chiefs of the family retained power down to the eighteenth century, despite their involvement in the various rebellions, but were eventually dispossessed. Even today, Pobal O'Keeffe is still the area in which the name is most common, with surrounding areas of Co Cork also including many of the name. It remains relatively rare outside that county.

O'Mahony, the most common contemporary form of the name, comes from the Irish O Mathghamhna,stemming, like MacMahon, from mathghamhan, meaning 'bear'. The surname was adopted in the eleventh century by one of the dominant families of the Munster Eoghanacht peoples, the Cineal Aodha; the individual from whom the name derives was the child of a marriage between Cian, chief of the Cineal Aodha, and Sadhbh, daughter of Brian Boru. With the rise of the MacCarthys in the twelfth century the influence of the O'Mahonys declined, and was largely confined to the two areas of west Cork with which they are still most strongly associated, the Iveagh peninsula and the barony of Kinalmeaky, around the modern town of Bandon. In these areas they retained a large measure of power and wealth until the final collapse of Gaelic power in the wars of the seventeenth century. The most famous modern bearer of the name was Eoin ('the Pope') O'Mahony (1904-1970), barrister and genealogist, who preserved and interpreted with accuracy and enthusiasm the traditions of his own and many other families, founding and organising the annual clan gathering of the O'Mahonys.

O'Neill is in Irish O Neill, from the personal name Niall, possibly meaning 'passionate' or 'vehement'. A clear distinction needs to be kept in mind between the family bearing this surname and the Ui Neill, the powerful tribal grouping claiming descent from Niall of the Nine Hostages, the fifth century monarch supposedly responsible for kidnapping St Patrick to Ireland. Out of the Ui Neill came many other well-known surnames, including O'Doherty, O'Donnell, O'Hagan and others. Within the Ui Neill the two principal sub-groups were the Cineal Eoghain and the Cineal Conaill, claiming descent from two of the sons of Niall, Eoghan and Conall respectively. The O'Neills were the leading family of the Cineal Eoghain, ruling the ancient territory of Tir Eoghain, comprising not only the modern Co Tyrone, but also large parts of Derry and Donegal. The first to use the name in recognisable hereditary fashion was what is now east Limerick, and died in 980. In the late twelfth century, as a result of the vicious struggle between the MacCarthys and the O'Briens for dominance in Munster, the O'Donovans were forced to migrate into the neighbouring county of Cork. There they gave the name of their kingdom to the modern barony of Carberry. Their territory comprised a large portion of this area reaching from the southeast coast almost as far as the modern town of Bantry. Their principal seat was at Castledonovan, in the centre of Drimoleague parish. The family remained powerful and prominent in the area down to the seventeenth century, when they played an important role in the defence of the Catholic and Gaelic Irish against the Cromwellian and Williamite campaigns. Like so many other members of the native aristocracy, the chiefs of the family were dispossessed in the punitive confiscation’s of the end of that century, but Colonel Daniel O'Donovan, the head of the family at that time, managed to regain some property in the area after the Treaty of Limerick, and re-established the family seat at Bawnlahan in the parishes of Myross and Vastlehaven. From him descends the current Chief of the Name, Daniel O'Donovan of Hollybrook, Skibbereen, Co Cork, the O'Donovan, recognised as such by the Chief Herald of Ireland. The most famous bearer of the name was John O'Donovan (1809-1861), founder of the Irish Archaeological Society, who virtually single-handedly laid the foundation for all subsequent study of Irish genealogy, history, language and topography.

O'Rourke comes from the Iris O Ruairc, from Ruarc, a personal name derived from the Old Norse Hrothekr (whence also 'Roderick'), meaning 'famous king'. Further more influence is seen in the frequency in the family of such names as Lochlann, Amhlaoibh (Olaf) and Sitric. The O'Rourkes were of the same stock as the O'Connors of Connacht, part of the large tribal grouping of the Ui Briain, claiming common descent from Brion. a fifth-century King of Connacht. In the early Middle Ages, the O'Connors and the O'Rourkes were engaged in a long and bloody struggle for supremacy in Connacht, a struggle which ended in the victory of the O'Connors. The Ruarc from whom the surname derives was a ninth-century King of Breifne, an area covering most of the modern counties of Leitrim and Cavan, along with part of Co Longford. The first to use his name as part of an hereditary surname was his grandson, Sean Fearghal O Ruairc, who died in 964. Over the following century and a half, four O'Rourkes were Kings of Connacht. After the twelfth century, they appear to have accepted the overlordship of the O'Connors, however reluctantly. They also had persistent problems with the other pre-eminent family of Breifne, the O'Reillys, which ultimately resulted in their territory being much reduced. The main stronghold of the family was at Dromahair, on the shores of Lough Gill in Co Leitrim. In common with most of the other ruling families of Gaelic Ireland, the O'Rourkes lost all of their possessions in the great upheavals of the sixteenth and seventeenth centuries. However, the line of descent from the last Chief of the Name, Brian Ballgh O'Rourke, who was inaugurated in 1529 and died in 1562, remains intact. The present holder of the title 'O Ruairc of Breifne', recognised as Chief of his Name by the Chief Herald of Ireland is Philip O Rorke, resident in London.

O'Shea, Shea and (O')Shee are anglicisations of the Irish O Seaghdha, from the personal name Seaghdha, meaning either 'hawk-like' or 'fortunate'. The surname arose in south Kerry, on the Iveragh peninsula, where the family held power in the early Middle Ages. Despite the later decline in their influence, they were not displaced, remaining extremely numerous in their original homeland down to the present day. The surname is also found in some numbers in counties Tipperary and Kilkenny. These are the descendants of family numbers who migrated north as early as the fourteenth century. They became prominent in Kilkenny especially, where the name was more often anglicised (O')Shee.
The most famous of the name in Irish history was Katherine O'Shea, mistress and later wife of Charles Stewart Parnell; their love affair brought about Parnell's downfall and changed the course of Irish history.

The original Irish is O Suileabhain, deriving from suil (eye). The dispute over the meaning of the remainder of the name is understandable, since the two principal alternatives are 'one-eyed' or 'hawk-eyed'. In Irish mythology, they are part of the Eoghanacht tribal grouping, descended, along with such prominent families as the MacCarthys and O'Callaghans, from the mythical Eoghan, supposedly one of the original Gaelic invaders. In historical times the O'Sullivans split into two major branches, the O'Sullivan Mor, based on the shores of Kenmare Bay in Co Kerry, and the O'Sullivan Beare, around Bantry and the Beara peninsula in Co Cork. Cork and Kerry are the areas in which popular tradition places the earliest Gaelic settlements, and even today, four out of five families of the name still live in the two counties, where it is the single most common surname.

O'Toole, along with Toole, comes from the Irish O Tuathail. This derives from the personal name Tuathal, meaning 'ruler of the people', used by many Irish kings and heroes and accordingly incorporated into a surname in a number of distinct area, among them south Ulster, Mayo and Kildare. Today the vast majority of those bearing the name are descended from the Kildare O'Tooles. The individual from whom the surname is taken was Tuathal, King of Leinster, who died c.958; the first to use the surname in true hereditary fashion appears to have been his grandson Doncaon, slain at Leighlin in 1014. Although the original territory of the O'Tooles lay in Co Kildare, in the twelfth century they were displaced by the invading Normans and migrated into the adjoining county of Wicklow, where the area they controlled was roughly identical to the old diocese of Glendalough, with the centre of their power in the region around the Glen of Imaal. Despite the proximity of Dublin, the centre of English rule in Ireland, the O'Tooles maintained a fierce independence and, together with their neighbours and occasional allies the O'Brynes, were a source of great fear to the inhabitants of Dublin and the Pale for almost four centuries. It was only in the seventeenth century, with the final and general collapse of Gaelic power, that the O'Tooles were 'pacified', as the English put it.
Unlike most of the other Gaelic aristocracy, however, the line of the O'Tooles survived intact; there were two branches, of Powerscourt and Castle Kevin, both in Wicklow. Descendants of the former are living in Wicklow and in the U.S. The representatives of the latter have lived in France for many generations. The most famous bearer of the name is undoubtedly St Laurence O'Toole, a member of the leading O'Toole family who became abbot of the monastery of Glendalough at the age of 25, and was chosen by the people and clergy as first Archbishop of Dublin in 1162. He subsequently led the resistance and negotiation with the Norman invaders.