Source : Webster's Revised Unabridged Dictionary (1913)

Irrevocable \Ir*rev"o*ca*ble\, a. [L. irrevocabilis: cf. F.
   irr['e]vocable. See {In-} not, and {Revoke}, and cf.
   {Irrevocable}.]
   Incapable of being recalled or revoked; unchangeable;
   irreversible; unalterable; as, an irrevocable promise or
   decree; irrevocable fate.

         Firm and irrevocable is my doom.         --Shak.
   -- {Ir*rev"o*ca*ble*ness}, n. -- {Ir*rev"o*ca*bly}, adv.

Source : WordNet®

irrevocably
     adv : in an irrevocable manner