The Irish Republican Brotherhood (IRB) was one of the most important revolutionary organisations within Irish history and had a profound influence on the emergence of Ireland as we understand it today. Through sixty years of secretive conspiracy the IRB bestrode Irish history like a colossus. It was a key factor in the emergence of cultural and political organisations in the late nineteenth and early twentieth centuries, played an active role in the Irish revolution of 1916-21 and was a crucial group in terms of the negotiation of the Anglo-Irish Treaty of 1921 and the emergence of the Irish Free State.

Colloquially known as the Fenians (this name was only officially used by the Fenian Brotherhood), the IRB conspired to overthrow British rule in Ireland and replace it with a democratic and secular Irish Republic.