The Irish Republican Brotherhood (IRB; Irish: Bráithreachas Phoblacht na hÉireann) was a secret oath-bound fraternal organisation dedicated to the establishment of an "independent democratic republic" in Ireland between 1858 and 1924. Its counterpart in the United States of America and Canada was initially the Fenian Brotherhood, but from the 1870s it was Clan na Gael. The members of both wings of the movement are often referred to as "Fenians". The IRB played an important role in the history of Ireland, as the chief advocate of republicanism during the campaign for Ireland's independence from the United Kingdom, successor to movements such as the United Irishmen of the 1790s and the Young Irelanders of the 1840s.
As part of the New Departure of the 1870s–80s, IRB members attempted to democratise the Home Rule League and its successor, the Irish Parliamentary Party, as well as taking part in the Land War. The IRB staged the Easter Rising in 1916, which led to the establishment of the first Dáil Éireann in 1919. The suppression of Dáil Éireann precipitated the Irish War of Independence and the signing of the Anglo-Irish Treaty in 1921, ultimately leading to the establishment of the Irish Free State, which excluded the territory of Northern Ireland.