Recorded as Cosgrave, Cosgrove, O' Cosgrave, MacCosgrove, McCosker, McCusker, O' Cosker, Cosker and Cusker, and recorded mainly in Ireland, but sometimes in Scotland, this is a surname of early Gaelic origins. Today it is regarded as being a branch of the Clan MacGuire, and has its epicentre at the village of Ballymacosker in County Fermanagh. It probably originates from the pre 10th century word 'coscrach' meaning victorious, and as such this would have been applied to the first chief of the clan. As to who this was, is uncertain, but it is likely that it originates from County Galway where Coningus O' Coscraich was the bishop of Clommacnois in the 10th century. In medieval times the spelling seems to have been as MacCoscraich, MacCogruyr and MacKuesker, being recorded later as Cosgrave, in Limerick as Cosgree, MacCosker and MacCusker, in Petty's Census of Ireland in 1659, and in County Tyrone as MacIlcosker. William Cosgrave was the first Taoiseach of the Irish State in 1922, and his son Liam held the same post in 1973 - 1976.

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