The Fenian Brotherhood was an Irish republican organisation founded in the United States in 1858 by John O'Mahony and Michael Doheny. It was a precursor to Clan na Gael, a sister organisation to the Irish Republican Brotherhood (IRB). Members were commonly known as "Fenians". O'Mahony, who was a Gaelic scholar, named his organisation after the Fianna, the legendary band of Irish warriors led by Fionn mac Cumhaill.

Irish republicanism is the political movement for the unity and independence of Ireland under a republic. Irish republicans view British rule in any part of Ireland as inherently illegitimate. An ideology since the 17th century, various methods have been employed to achieve the republic, including rebellions and paramilitary campaigns. Although the makeup of republicanism has been multidenominational, its relation to catholicism increasingly became central.