Recorded in several spellings including McGarrahan, McGarahan, McGaraghan, Garahan, Garaghan Garraghan and possibly others, this is a Gaelic-Irish surname. It was originally spelt Mag Arachain, to which surprisingly the famous book "The surnames of Ireland" by the late Edward MacLysaght does not offer a meaning. This always seems to happen when the possible translation is either rude or lude. In fact this name is part way there, and describes a fearsome eater, or to be precise "The son of the son of the great eater"! Most Irish surnames, and indeed most Gaelic surnames, developed from a nickname for the original chief of the clan or sept. This usually extolled the virtues or otherwise of being a great warrior or the follower of a saint, but occasionally the real truth came out. The original chief of the famous O' Kennedys was called "ugly head," so by comparison to be called "The Great Eater" is complimentary and no doubt was intended to be at the time it was given around the 12th century. The first known recording is that of Owen MacGarahan of County Galway in 1577, and it is said that the clan were originally from County Fermanagh.