The word Fenian served as an umbrella term for the Irish Republican Brotherhood (IRB) and their affiliate in the United States, the Fenian Brotherhood. They were secret political organisations in the late 19th and early 20th centuries dedicated to the establishment of an independent Irish Republic. In 1867 they sought to coordinate raids into Canada from the United States with a rising in Ireland. In the 1916 Easter Rising and the 1919–1921 Irish War of Independence, the IRB led the republican struggle.

The Irish Republican Brotherhood 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 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.