member of an Irish nationalist secret society active chiefly in Ireland, the United States, and Britain, especially during the 1860s. The name derives from the Fianna Eireann, the legendary band of Irish warriors led by the fictional Finn MacCumhaill (MacCool). The society was founded in the United States by John O'Mahony and in Ireland by James Stephens (1858). Plans for a rising against British rule in Ireland miscarried, but the American Fenians staged abortive raids across the border into British Canada in 1866, 1870, and 1871 and were a cause of friction between the U.S. and British governments.

The Irish wing of the society was sometimes called the Irish Republican Brotherhood, a name that continued to be used after Fenianism proper had virtually died out in the early 1870s. Arthur Griffith, a member of the Brotherhood, founded the Irish nationalist party Sinn Féin (“We Ourselves”) in 1905.