The Collaborative International Dictionary of English v.0.48:

irredentist \irredentist\ n.
   an advocate of irredentism, especially a person in one
   country who advocates the incorporation of a region in
   another country related historically to his country.

   Syn: irridentist.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

irredentist
    n 1: an advocate of irredentism [syn: irredentist,
         irridentist]