The Collaborative International Dictionary of English v.0.48:

Irretrievable \Ir`re*triev"a*ble\, a.
   Not retrievable; irrecoverable; irreparable; as, an
   irretrievable loss.

   Syn: Irremediable; incurable; irrecoverable.
        [1913 Webster]

WordNet (r) 3.0 (2006):

irretrievable
    adj 1: impossible to recover or recoup or overcome; "an
           irretrievable loss"; "irretrievable errors in judgment"
           [syn: irretrievable, unretrievable]

Moby Thesaurus II by Grady Ward, 1.0:

77 Moby Thesaurus words for "irretrievable":
   ablated, beyond recall, beyond remedy, by the board, changeless,
   constant, consumed, cureless, depleted, dissipated, eroded,
   expended, forfeit, forfeited, gone, immedicable, immutable,
   incommutable, inconvertible, incorrigible, incurable, indefeasible,
   inert, inoperable, insusceptible of change, intransmutable,
   invariable, irreclaimable, irrecoverable, irredeemable,
   irreformable, irremediable, irreparable, irreversible, irrevocable,
   lasting, long-lost, lost, lost to, noble, nonreturnable,
   nonreversible, out the window, past hope, past praying for,
   permanent, remediless, reverseless, ruined, shrunken, squandered,
   terminal, unalterable, unalterative, unaltered, unchangeable,
   unchanged, unchanging, uncorrectable, undeflectable, undeviating,
   undone, unmitigable, unmodifiable, unrelievable, unremitting,
   unrestorable, unreturnable, unsalvable, unsalvageable,
   unsusceptible, unvariable, unvarying, used, used up, wasted,
   worn away