fenian

1. a member of the Irish Republican Brotherhood, a 19th-century revolutionary nationalist organization among the Irish in the US and Ireland. The Fenians staged an unsuccessful revolt in Ireland in 1867 and were responsible for isolated revolutionary acts against the British until the early 20th century, when they were gradually eclipsed by the IRA.
2. (chiefly in Northern Ireland) a derogatory term for a Catholic or Irish nationalist.

from Old Irish féne, the name of an ancient Irish people, confused with fíann, fianna (see Fianna Fáil).