McMahon or MacMahon ( mək-MAN or mək-MAHN; Irish: Mac Mathúna, traditionally Mac Mathghamhna; meaning "son of the bear") is an Irish surname. The surname came into use around the 11th century by two different Irish clans: The MacMahons of Thomond, a sept of the Dál gCais, and the MacMahons of Oriel, rulers of Airgíalla. Additionally, a separate McMahon family in County Fermanagh is descended from Mahon Maguire, a grandson of Donn Carrach Maguire. According to historian C. Thomas Cairney, the Ma…