The principal Munster sept of the name Callaghan were lords of Cineál Aodha in South Cork originally.
The family were dispossessed of their ancestral home and 9,700 ha (24,000 acres; 97 km2) by the Cromwellian Plantation and settled in East Clare.
[5] An entirely different sept, Ó Ceileacháin in Irish, is to be found in the counties Armagh, Louth, Meath and Monaghan.
In County Meath, where it is widespread but has been found mainly in the parishes of Kells, Trim and Athboy, it is mainly anglicised as Callahan, Callaghan or O'Callaghan (with local spelling variants).
[6] Members of the Ó Ceileacháin family were mentioned in the Annals of the Four Masters as being lords of Uí Breasail, a district on the southern shore of Lough Neagh, and priors of Armagh in the 11th century.