MacMahon, MacMathghamhna, Mac Mathúna
A Gaelic surname meaning son of Mathghamhna, which personal name meant ‘bear-calf’. A sept of Thuas Mhumhan (Thomond), they derive their descent from Mahon, son of Murtagh More ó Briain, High King (1094-1119). Their territory was Corcu Baiscinn, which comprised the baronies of Moyarta and Clonderalaw in south-west Clare. A sept of Orghialla (Oriel), who replaced the ó Cearbhaill (O’Carroll) in the thirteenth century as lords of Orghialla. They remained lords of Orghialla down to the reign of Elizabeth I and were powerful in Monaghan until the time of Cromwell. Also see Mathews. In 1890, MacMahon was the sixty-fourth most numerous surname, the estimated number of bearers was 10,700 and was the most common surname in Clare. In that year the surname was chiefly found in Clare, Monaghan, Limerick and Dublin. In the United States McMahon is the 990th most numerous surname with an estimated 33,000 bearers, and McMahan is the 2,493rd most numerous surname with an estimated 13,750 bearers, and it is numerous in the state of Connecticut.