The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

         Firm and irrevocable is my doom.         --Shak.
   -- Ir*rev"o*ca*ble*ness, n. -- Ir*rev"o*ca*bly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

irrevocably
    adv 1: in an irrevocable manner

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "irrevocably":
   absolutely, certainly, clean, fatefully, indefeasibly, ineluctably,
   inescapably, inevasibly, inevitably, inexorably, inflexibly,
   irreclaimably, irrecoverably, irredeemably, irremediably,
   irreparably, irretrievably, irreversibly, necessarily, perfectly,
   plain, plumb, quite, relentlessly, right, sheer, stark, surely,
   unavoidably, uncontrollably, unpreventably, unrelievedly,
   unsalvably, unsalvageably, unyieldingly