McMahon, also spelt MacMahon (older Irish orthography: ; reformed Irish orthography: ), is a surname of Irish origin. It is derived from the Gaelic Mac Mathghamhna meaning 'son of the bear'. The surname came into use around the 11th century and is used by two different Irish clans, also known as septs: The MacMahons of Thomond (County Clare) from Mid-West of the Republic of Ireland and the MacMahons of Oriel (County Monaghan) from North of the Republic of Ireland. The MacMahons of Thomond (County Clare) The Thomond MacMahons were part of the great tribal grouping, the Dál gCais, and claimed descent from Mahon O'Brien, son of Muirchertach Ua Briain, High King of Ireland. Corcu Baiscin was held by the descendants of Carbry Bascain until the 11th Century, when the descendants of Mahon O’Brien conquered them. According to Frost, Mahone 'a quo MacMahon' died in 1129, leaving two sons Murtagh and Dermot, with Murtagh being the ancestor of the main line of McMahons in Clare. The McMahons seized the Corcabaskin territory in the south of what is now County Clare in the 12th century about the same time as they adopted the fixed surname. Quoting the "Annals of the Four Masters", Frost says that Donogh MacMahon, Lord of Corcabaskin, died in 1488 and two MacMahons, Brian and Teige Roe, were established in his place – Brian in West Corcabaskin (known as Moyarta) and Teige Roe in the East (known as Clonderalaw). The two ruling branches of the clan became firmly established in Corcabaskin, West Clare, where their once strongholds, Carrigaholt Castle and Clonderlaw Castle, are prominent landmarks and a source of local interest today. In August 1585, the Irish leaders of Thomond were forced to sign an Indenture with Sir John Perrott, the English Lord Deputy of Ireland. Frost says that "some of the signatories of the Deed of Composition seem to have been bribed into conformity by Perrott ... [including Teige MacMahon ... of Clonderalaw [and] Turlough MacMahon ... of Moyarta". They were allowed to retain their castles and lands free of crown rent. The two MacMahons would have received the English titles of Baronet, replacing their traditional Irish titles, about this time. The two sons who succeeded them as heads of their families certainly each carried the English title. The last chief of the West Corcabaskin MacMahons, Sir Teige Caech MacMahon, was killed at or shortly after the battle