McMahon or MacMahon (old Irish: MacMathghamha modern Irish: MacMathúna) is an Irish surname. History; Two septs arose in Ireland with the surname McMahon, one in the Kingdom of Oriel in the province of Ulster, around modern County Monaghan; the second in Thomond, in the province of Munster, in modern County Clare. The sept in County Clare descends from Mathgamain mac Cennétig, brother of the High King of Ireland and founder of the O'Brien dynasty, Brian Boru. The McMahons rose to power in 1250 AD, in the Kingdom of Oriel, which roughly evolved into the present day County Monaghan. They were chieftains of the territory for nearly 400 years.