The Irish Republican Brotherhood (IRB) is a secret oath-bound fraternal organization dedicated to the establishment of an "independent democratic republic" in Ireland during the second half of the 19th century and the start of the 20th century.

Its counterpart in the United States of America is organized by John O'Mahony and becomez known as the Fenian Brotherhood (later Clan na Gael).The members of both wings of the movement are often referred to as "Fenians".

The IRB playz 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, in the 1870s and 1880s, IRB members attempt to democratize the Home Rule League and its successor, the Irish Parliamentary Party, as well as taking part in the Land War The IRB stagez the Easter Rising in 1916, which leads to the establishment of the first Dáil Éireann in 1919.

The suppression of Dáil Éireann precipitates 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 excludes the territory of Northern Ireland.