Clanmahon (Irish: Clann Mhathúna[1]) is a barony in County Cavan, Ireland.

[2][3][4] Clanmahon takes its name from the Irish Clann Mathúna, originally Cloinne Mathghamhna, "Mathgamhain's tribe," the ruling Gaelic Irish dynasty in the area in the Middle Ages.

[1] Clanmahon is the southern part of County Cavan, east of the River Erne and north of Lough Sheelin.

[1] The descendants of Tomas Mór O'Reilly, king of East Breifne 1384–92, were rulers of Clanmahon in the 15th century; Tomás was the son of Mathgamain Ua Raighillaigh.

[6] According to the Annals of the Four Masters, in 1537, "The son of O'Reilly (Brian, the son of Farrell), a great loss in his own country, was slain by the people of the English Lord Justice [Leonard Grey, 1st Viscount Grane], who came to commit ravages in Clann-Mahon.