The number of people is counted during each census. In the Republic of Ireland there is a full census every 10 years and a limited census halfway between the full census. The last full census was in 2002. For the census each house receives a form to fill up. Everybody fills in the number of people in the house at a particular time plus some other information.

County Size (kmē) Population Number per kmē Dublin 921 1,122,600 1,219 Antrim 2844 566,400 199 Cork 7,457 448,181 60 Down 2448 416,600 170 Derry* 2074 214,800 104 Galway 6,148 208,826 34 Limerick 2,686 175,529 65 Kildare 1,693 163,995 97 Tyrone 3155 161,800 51 Tipperary 4,303 140,281 33 Donegal 4,841 137,383 28 Meath 2,342 133,936 57 Kerry 4,746 132,424 28 Armagh 1254 127,700 102 Mayo 5,585 117,428 21 Wexford 2,352 116,543 50 Wicklow 2,024 114,719 57 Clare 3,147 103,333 33 Louth 820 101,802 124 Waterford 1,837 101,518 55 Kilkenny 2,061 80,421 39 Westmeath 1,838 72,027 39 Offaly 2,000 63,702 32 Laois 1,719 58,732 34 Sligo 1,836 58,178 32 Cavan 1,931 56,416 29 Fermanagh 1691 54,200 32 Roscommon 2,547 53,803 21 Monaghan 1,294 52,772 41 Carlow 896 45,845 51 Longford 1,091 31,127 29 Leitrim 1,588 25,815 16

* Derry or Londonderry. Both names are in use.

Sources:

Republic of Ireland - 2002 Census results
Northern Ireland - Population estimates 1992 per Microsoft Encarta 1996