By contrast, the term Sept is virtually unknown to the Irish general public, and is found solely in the dustier history books in the larger libraries. It is a term used almost exclusively by a small minority of academics who appear to have latched onto it because it was used by two highly regarded historians: Eoin MacNeill and Edward MacLysaght. The following scholarly analysis by Kenneth W. Nicholls (lecturer in the Department of History at University College Cork and editor for the Irish Manuscripts Commission) provides an insight into the importance of the clan in Medieval Ireland, and clarifies the background to the usage of terms like Clan and Sept:-

"Medieval Ireland was, of course, a society of clans or lineages - referred to as 'nations' in contemporary English terminology - and the most outstanding feature in the Gaelicization of the Anglo-Norman settlers was the speed with which, within the first century following the invasion, the concept of the clan had become established among them. Irish scholars have shown a curious dislike for the word 'clan', itself an Irish word (clann, lit. 'children', 'offspring') borrowed through Scotland, but as the term is in normal use by social anthropologists to denote the kind of corporate descent group of which I am speaking, I have no hesitation in employing it. The study, however, of clan- or lineage-based societies - which, whether in Medieval Ireland, in Asia or Africa, constitute a particular form of organisation with distinctive features in common - is comparatively recent. In the sense in which I am using it here, a clan may be defined as a unilineal (in the Irish case, patrilineal) descent group forming a definite corporate entity with political and legal functions. This latter part of the definition is an important one, for the functions of the clan in a clan-organised society lie entirely in the 'politico-jural' and not in the 'socio-familial' sphere, that is to say, they are concerned with the political and legal aspects of life and not with those of the family. The earlier term for such a unit was fine, which by late medieval times had been replaced in Ireland (although it survived in Gaelic Scotland) by the term sliocht (literally, 'division') translated into Renaissance Romance-English as 'sept'. Normally a clan would occupy and possess particular lands or territory, its occupation or ownership of the land being one of its most important corporate functions. (This does not, it need hardly be said, imply that the territory was held in common among the members of the clan or that outsiders would not be present within the clan territory. The objection of some Irish scholars to the concept of the clan may owe its origin to a reaction against absurdities of this kind.) As the clan is a corporate entity with functions only in particular spheres and aspects of life it is, of course, absurd to conceive of a clan-based society as being divided into clans as if into compartments; then clan, like a modern company, can be a very variable thing. A clan may be represented by a single individual only, the only member remaining of his descent-group, which nevertheless continues to exist so long as any member of it survives. The small descent-groups within a larger clan may each constitute entities or clans, while remaining part of the larger one, and may again be similarly divided themselves."

"In the case of Ireland, the greater part of the humbler classes certainly did not belong to any recognised clans or descent-groups other than their immediate family groups (father and sons, or a group of brothers). In the case of persons like these, devoid of political influence or property, the clan would have had no functions which could serve to hold it together. Conall Mageoghegan, writing in 1627, refers contemptuously to persons of this sort as 'mere churls and labouring men, [not] one of whom knows his own great-grandfather'. The phrase is significant; in a lineage-based society the keeping of genealogies is of primary importance. Not only is membership of the clan conferred by descent, but the precise details of this descent may determine a person's legal rights in, for instance, the property of the clan. In Ireland the keeping of genealogies was entrusted to the professional families of scribes and chroniclers. In 1635 we find a genealogy of the Butlers of Shanballyduff in County Tipperary prepared by Hugh Óg Magrath 'out of the new and old books of his ancestors written in the Irish language', and in 1662 Arthur O Neill, about to be admitted as a knight of the Order of Calatrava in Spain and asked for his pedigree, referred the Order to 'the chronicler Don Tulio Conrreo', otherwise Tuileagna O Mulconry, who duly produced the required pedigree back to Donnell of Armagh, King of Ireland in 976."

"As the clan was a unit only in a legal and political sense, one must not, of course, expect it to show the sort of internal solidarity one expects of the family. Indeed, causes of tension and conflict might be expected to be highest within the lineage group, where rights over the clan property would be a constant ground for dispute. When we read in an early seventeenth-century law suit, with reference to two Purcell brothers who held in common a minute property in County Tipperary, that 'the said Patrick was killed by the said Geoffrey for some difference betwixt them about the said land', we see what must have been a common outcome of fraternal tension. Where the succession to a great lordship was at stake, violence of this kind would be even more likely and cousins, whose interests would normally be in direct opposition, would be almost automatic enemies. The clan might close its ranks against an outsider and collectively seek vengeance against the slayer of one of its members, but within itself it might equally exist in a permanent state of hostility and division. Such hostility, if continued over generations, would inevitably lead to its division into separate fragments, each of which would function as a separate clan, and the more numerous the clan, the sooner was this likely to happen."

NICHOLLS, K.W.
Gaelic and Gaelicized Ireland in the Middle Ages
Dublin: Lilliput Press, Second Edition 2003
pp. 8-11