The Collaborative International Dictionary of English v.0.48:

Fenian \Fe"ni*an\, n. [From the Finians or Fenii, the old
   militia of Ireland, who were so called from Fin or Finn,
   Fionn, or Fingal, a popular hero of Irish traditional
   history.]
   A member of a secret organization, consisting mainly of
   Irishmen, having for its aim the overthrow of English rule in
   Ireland.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Fenian \Fe"ni*an\, a.
   Pertaining to Fenians or to Fenianism.
   [1913 Webster]