The surname McDonagh, also spelled MacDonagh is from the Irish language Mac Dhonnchadha, and is now one of the rarer surnames of Ireland. Mac Dhonnchadha, Mac Donnchadha, Mac Donnacha or Mac Donnchaidh is the original form of McDonagh These surnames are found in their greatest numbers in Connacht (Connaught), especially the counties of Sligo, Roscommon and Galway. Dhonnchadha, literally translated from the Irish language, means "brown warrior". The prefix mac means "son of".