Mac Cathail comes from the personal name Cathal. Cahill is one of the oldest recorded surnames in Ireland. There were several different clans of the name, located in Kerry, Galway, Tipperary and Clare. The Galway sept of the clan was initially very prominent, however their power declined, and the southern branches of the families flourished. The name is one of the 100 most numerous surnames in Ireland, and is number one in the province of Munster.