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):

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

Moby Thesaurus II by Grady Ward, 1.0:

106 Moby Thesaurus words for "irrevocable":
   absolute, beyond recall, beyond remedy, binding, certain,
   changeless, compulsory, conclusive, constant, cureless, decisive,
   decretory, dictated, enduring, entailed, established, everlasting,
   fated, fateful, final, fixed, gone, hard-and-fast, immedicable,
   immutable, imperative, imposed, incommutable, inconvertible,
   incorrigible, incurable, indefeasible, ineluctable, inert,
   inescapable, inevasible, inevitable, inexorable, inflexible,
   inoperable, insusceptible of change, intransmutable, invariable,
   irreclaimable, irrecoverable, irredeemable, irreformable,
   irremediable, irreparable, irresistible, irretrievable,
   irreversible, lasting, lost, mandated, mandatory, must, necessary,
   noble, nonreturnable, nonreversible, obligatory, past hope,
   past praying for, peremptory, permanent, prescript, prescriptive,
   relentless, remediless, required, resistless, reverseless, ruined,
   settled, sure, sure as death, sure as fate, terminal, ultimate,
   unalterable, unalterative, unaltered, unavoidable, unchangeable,
   unchanged, unchanging, uncontrollable, undeflectable, undeviating,
   undone, unmitigable, unmodifiable, unpreventable, unrelievable,
   unremitting, unrestorable, unreturnable, unsalvable, unsalvageable,
   unstoppable, unsusceptible, unvariable, unvarying, unyielding,
   without appeal