McMahon, also spelt MacMahon (older Irish orthography: Mac Mathghamhna; reformed Irish orthography: Mac Mathúna), were different Middle Age era Irish clans. Their name is derived from the Gaelic Mac Mathghamhna meaning 'son of the bear'. According to historian C. Thomas Cairney, the MacMahons were one of the chiefly families of the Dal gCais or Dalcassians who were a tribe of the Erainn who were the second wave of Celts to settle in Ireland between about 500 and 100 BC. The same historian stated that another group of McMahons were a chiefly family of the Oirghialla or Airgíalla tribe who were in turn from the Laigin tribe who were the third wave of Celts to settle in Ireland during the first century BC.
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Mac Mathghamhna in the context of County Monaghan
County Monaghan (/ˈmɒnəhən/ MON-ə-hən; Irish: Contae Mhuineacháin) is a county in Ireland. It is in the province of Ulster and is part of Border strategic planning area of the Northern and Western Region. It is named after the town of Monaghan. Monaghan County Council is the local authority for the county. The population of the county was 65,288, according to the 2022 census.
The county has existed since 1585 when the Mac Mathghamhna rulers of Airgíalla agreed to join the Kingdom of Ireland. Following the 20th-century Irish War of Independence and the signing of the Anglo-Irish Treaty, Monaghan was one of three Ulster counties to join the Irish Free State rather than Northern Ireland.
View the full Wikipedia page for County Monaghan