The word Fenian () served as an umbrella term for the Irish Republican Brotherhood (IRB) and their affiliate in the United States, the Fenian Brotherhood, secret political organisations in the late 19th and early 20th century 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 term Fenian today occurs as a derogatory sectarian term in Ireland, referring to Irish nationalists or Catholics, particularly in Northern Ireland. The term has been used similarly in Scotland by Protestants as an attempt at a derogatory religious slur when referring to Scottish Catholics or to Scots with Irish ancestry.