Callaghan - Ó Ceallacháin The derivation from ceallach, strife, which usually given, is questioned but no acceptable alternative has been suggested. The eponymous ancestor in this case was Ceallacháin, King of Munster (d. 952). The sept was important in the present Co. Cork until the seventeenth century and the name is still very numerous there. The chief family was transplanted under the Cromwellian regime to east Clare, where the village of O’Callghan’s Mills is called after them.
Campbell - Mac Cathmhaoil (cathmhaoil, battle chief). An Irish sept in Tyrone; in Donegal it is usually of Scottish galloglass origin, viz. Mac Ailín a branch of the clan Campbell (whose name is from cam béal, crooked mouth) Many Campbells are of more recent Scottish immigrants. See MacCawell. The name has been abbreviated to Camp and even Kemp in Co. Cavan.
MacCarthy - Mac Carthaigh (cárthach, loving). The chief family of the Eoghanacht and one of the leading septs of Munster, prominent in the history of Ireland from the earliest times to the present. MacCarthy is the most numerous Mac name in Ireland.
Cassidy - Ó Caiside A Fermanagh family of ollavs and physicians to the Maguires. Now numerous in all the provinces except Connacht.
Clery - Cleary Ó Cléirigh (cléireach, clerk) One of the earliest hereditary surnames. Originally of Kilmacduagh (Co. Galway) the sept was dispersed and after the thirteenth century settled in several parts of the country; the most important branch were in Donegal where they became notable as poets and antiquaries. In modern times the name is found mainly in Munster and Dublin.
O'Connor - ÓConchobhair. The name of six distinct and important septs. In Connacht there were O’Connor and O’Conor Don (of which was the last High King of Ireland) with its branches O’Conor Roe and O’Conor Sligo; Also O’Conor Faly (i.e. of Offaly), O’Connor Kerry and O’Connor of Corcomroe (north Clare). The prefix, O, formerly widely discarded, has been generally resumed. Similarly the variant from Connors has been O’Connor again.
(O) Conroy - Conree, Conary, Conry. These mainly Connacht names, owing to the similarity to of the anglicized forms, have become virtually indistinguishable. They represent four Gaelic originals, viz. Mac Conraoi (Galway and Clare), Ó Conraoi (Galway), Ó Conaire (Munster and Ó Maolchonaire (an important literary family of Co. Roscommon)
Cooney - Ó Cuana (for the probable derivation see Coonan). Originally of Tyrone this family later migrated to north Connacht. The Cooneys of east Clare and south-east Galway may be of different origin.
MacCormack - Cormick Mac Cormaic. This like MacCormican is formed from the forename Cormac. This name is numerous throughout all the provinces, the spelling MacCormick being more usual in Ulster. For the most part it originated as a simple patronymic; the only recognized sept of the name was of the Fermanagh-Longford area. Many of the MacCormac(k) families of Ulster are of Scottish origin, being a branch of the clan Buchanan-MacCormick of MacLaine.
Daly - Dawley Ó Dálaigh (dálach, from dáil, assembly). One of the greatest names in Irish literature. Originally West Meath, but sub-septs in several different localities as Map. As that in Desmond appears in the records as early as 1165 it is probable that this was a distinct sept.
Darcy - Ó Dorchaidhe (dacha, dark) One of the ‘Tribes of Galway’ also anglicized Dorsey, it is the name of two septs, one in Mayo and Galway, the other in Co. Wexford.
(O) Delaney - Ó Dubhshláine (another case of dubh, black-Sláine, perhaps the river Slaney) The prefix O has been completely discarded in the anglicized form of the name. It appears as Delane in Mayo. Both now and in the past it is of Leix and Kilkenny.
(O) Dempsey - Ó Díomasaigh (díomasach, proud). A powerful sept in Clanmalier. O’Dempsey was one of the very few chiefs who defeated Strongbow in a military engagement. Many of his successors distinguished themselves as Irish patriots and they were ruined as a result of their loyalty to James II. The name is now numerous in all the provinces.
Disney - Derived from a French place-name and originally written D’Isigny etc., the name Disney occurs quite frequently in the records of several Irish counties in the south and midlands since the first half of the seventeenth century.
(O) Dolan - The general accepted form in Irish today is Ó Dúbhláin (mod. Ó Dúláin) as given by Woulfe and others. O’Dolean, later Dolan, derives from Ó Dobhailen the name of a family on record since the twelfth century in the baronies of Clonmacnowen, Co. Galway, and Athlone, Co. Roscommon, in the heart of the Uí Mainecountry and quite distinct from Ó Doibhilin (Devlin). There has been a movement north-eastwards so that now the name Dolan is numerous in Co. Leitrim, Fermanagh, and Cavan as well Co. Galway and Roscommon.