John O'Mahony (1816–1877) was founder of the Irish Republican Brotherhood.
John O'Mahony may also refer to: John O'Mahony (Mayo politician) (1953–2024), Irish Fine Gael politician and manager of the Galway Football Team John O'Mahony (Gaelic footballer) (1937–2012), Irish Gaelic footballer John O'Mahony (Australian footballer) (born 1931), Australian footballer for Hawthorn John O'Mahony (antiquarian) (born 1844), Irish Catholic priest and founder of the Cork Historical and Archaeological Society Sean Matgamna (born 1941), also known as John O'Mahony, Trotskyist theorist Seán O'Mahony (1864–1934), Sinn Féin politician who represented the South Fermanagh constituency John O Mahoney, Irish Gaelic footballer