Mahon

The MacMahons of Oriel (Irish Mac Mathghamhna, meaning son of the bear) were a Gaelic Irish chiefly family who ruled the kingdom of Airgíalla, anglicised as Oriel, centred on County Monaghan. A sept of Clann Nadsluaigh claiming descent from the Three Collas, they took their surname from Niall Mac Mathghamhna, descended from Mathgamna, lord of Fernmaige. They held the kingship of Oriel from 1243.

Caught between the Tudor crown and the O'Neills of Tír Eoghain, Rossa Buidhe MacMahon surrendered and regranted his lands in 1590, when Oriel became County Monaghan, divided among five baronial families.

The MacMahons joined the Irish rebellion of 1641 under Colla MacMahon; Hugh Oge MacMahon, betrayed after the Dublin Castle plot, escaped the Tower of London in 1644 but was recaptured and beheaded, after which the family's lands passed to Protestant settlers.