The Collaborative International Dictionary of English v.0.48:

Irrevokable \Ir`re*vok"a*ble\, a. [Pref. ir- not + revoke.]
   Irrevocable. [R.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

irrevokable
    adj 1: incapable of being retracted or revoked; "firm and
           irrevocable is my doom"- Shakespeare [syn: irrevocable,
           irrevokable] [ant: revocable, revokable]