The Collaborative International Dictionary of English v.0.48:

Orangeman \Or"ange*man\, n.; pl. -men.
   One of a secret society, organized in the north of Ireland in
   1795, the professed objects of which are the defense of the
   reigning sovereign of Great Britain, the support of the
   Protestant religion, the maintenance of the laws of the
   kingdom, etc.; -- so called in honor of William, Prince of
   Orange, who became William III. of England.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Orangeman
    n 1: a member of a society founded in Ireland in 1795 to uphold
         Protestantism and the British sovereign