The Collaborative International Dictionary of English v.0.48:

Brehon \Bre"hon\, n. [Ir. breitheamh judge.]
   An ancient Irish or Scotch judge.
   [1913 Webster]

   Brehon laws, the ancient Irish laws, -- unwritten, like the
      common law of England. They were abolished by statute of
      Edward III.
      [1913 Webster]