Mulcahy is a surname of Irish Gaelic origin. The anglicized form of "Ó Maolchatha" which in Gaelic means 'a descendant of a devotee of Cathach', a personal name meaning Warlike. The name is thought to originate in County Tipperary, However the earliest mention of the family appears in the Annals of Inisfallen in 1317 AD and subsequent references in and around the Churches of County Kerry in the 15th century.
While it is true that Cathach means Warlike, the inclusion of the prefix Maol refers to a 'devotee' of an individual whose name was Cathach. So the full name of Ó Maolchatha means 'A descendant of a devotee of Cathach' Cathach was most likely a saint or nickname for an early church figure in medieval Ireland.