Callahan, Calligan, Kealahan, Kelaghan, O’ Callaghan, O’ Kelaghan, Ó Ceallacháin
The O’Callaghan clan originates in the province of Munster in the south of Ireland. It is likely that the name came from Ceallachan, who was the king of Munster in the 10th century. The family was one of the most powerful in Munster, and was involved in a prolonged feud with one of the other dominant clans, the Mac Carthys. Today, the name is ranked as 43rd most numerous in Ireland, and is strongest in the province of Munster.