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.

Young Ireland was a political and cultural movement in the 1840s committed to an all-Ireland struggle for independence and democratic reform. Grouped around the Dublin weekly The Nation, it took issue with the compromises and clericalism of the larger national movement, Daniel O'Connell's Repeal Association, from which it seceded in 1847. Despairing, in the face of the Great Famine, of any other course, in 1848 Young Irelanders attempted an insurrection. Following the arrest and the exile of most of their leading figures, the movement split between those who carried the commitment to "physical force" forward into the Irish Republican Brotherhood, and those who sought to build a "League of North and South" linking an independent Irish parliamentary party to tenant agitation for land reform.