homelands in the Golden Vale of Tipperary by the expansion of the O'Brien sept in the middle of the twelfth century.

Carthach's son was known as Muireadhach mac Carthaigh (meaning "Muireadhach, son of Carthach"). Such ephemeral patronymics were common at the time. However, when Muireadhach died in 1092 his sons Tadhg and Cormac adopted Mac Carthaigh as an actual surname. Following the treaty of Glanmire in 1118,