The Collaborative International Dictionary of English v.0.48:

Irreversible \Ir`re*vers"i*ble\, a.
   1. Incapable of being reversed or turned about or back;
      incapable of being made to run backward; as, an
      irreversible engine; an irreversible process; an
      irreversible reaction.
      [1913 Webster +PJC]

   2. Incapable of being reversed, recalled, repealed, or
      annulled; as, an irreversible sentence or decree.
      [1913 Webster]

            This rejection of the Jews, as it is not universal,
            so neither is it final and irreversible. --Jortin.

   Syn: Irrevocable; irrepealable; unchangeable.
        [1913 Webster]

WordNet (r) 3.0 (2006):

irreversible
    adj 1: incapable of being reversed; "irreversible momentum
           toward revolution" [ant: reversible]

Moby Thesaurus II by Grady Ward, 1.0:

94 Moby Thesaurus words for "irreversible":
   beyond recall, beyond remedy, changeless, chronic, confirmed,
   constant, cureless, deep-dyed, deep-fixed, deep-rooted,
   deep-seated, deep-set, deep-settled, direct, dyed-in-the-wool,
   established, fast, final, fixed, gone, immediate, immedicable,
   immutable, implanted, incommutable, inconvertible, incorrigible,
   inculcated, incurable, indefeasible, inert, infixed, ingrained,
   inoperable, instilled, insusceptible of change, intransmutable,
   invariable, inveterate, irreclaimable, irrecoverable, irredeemable,
   irreformable, irremediable, irreparable, irretrievable,
   irrevocable, lasting, long-established, lost, noble, nonreturnable,
   nonreversible, one-way, past hope, past praying for, permanent,
   remediless, reverseless, rooted, ruined, set, settled,
   settled in habit, straight, straightaway, straightforward,
   straightway, terminal, thorough, unalterable, unalterative,
   unaltered, unbroken, unchangeable, unchanged, unchanging,
   undeflectable, undeviating, undone, unidirectional, uninterrupted,
   unmitigable, unmodifiable, unrelievable, unremitting, unrestorable,
   unreturnable, unsalvable, unsalvageable, unsusceptible, unswerving,
   unvariable, unvarying