High Quality Content by WIKIPEDIA articles! The Fenian Brotherhood was an Irish republican organization founded in the United States in 1858 by John O'Mahony and Michael Doheny.It was a precursor to Clan na Gael, a sister organization to the Irish Republican Brotherhood. Members were commonly known as "Fenians". The revolutionary society was founded by John O'Mahony in 1858. O'Mahony, who was a Celtic scholar, named his organization after the Fianna, the legendary band of Irish warriors led by Fionn mac Cumhaill.