The name McDonagh originated independently in two separate parts of the country; Connacht and Munster. It is a branch of the MacDermots of Connacht where the name is numerous. The family held power in counties Sligo and Roscommon. In County Cork, the MacDonagh’s were a branch of the MacCarthys. Nowadays, the name MacDonagh is very rare in that part of the country, with many of these MacDonaghs reverting to the MacCarthy name.