Monaghan, Monahan, ó Manacháin
A Gaelic surname meaning descendant of Manach, meaning ‘a monk’. A sept of Connacht, who were chiefs of Ui Briuin na Sionna in the barony of Ballintober in Roscommon. In the year 1249 they were ousted by the O’Beirnes. In 1890 the surname was principally found in Galway, Mayo, Dublin and Fermanagh, and the estimated number of bearers for Monaghan was 4,300 and for Monahan was 1,880.