Fenian

Concise Oxford English Dictionary © 2008 Oxford University Press:
Fenian /ˈfiːnɪən/

▶noun

• 1 a member of the Irish Republican Brotherhood, a 19th-century revolutionary nationalist organization among the Irish in the US and Ireland.

• 2 informal, offensive (chiefly in Northern Ireland) a Protestant name for a Catholic.

– derivatives
Fenianism noun.

– origin from Old Ir. féne, the name of an ancient Irish people, confused with fíann, fianna ‘band of warriors’, in particular the soldiers of the legendary king Finn MacCool.