The Collaborative International Dictionary of English v.0.48:

Irrecoverable \Ir`re*cov"er*a*ble\, a.
   Not capable of being recovered, regained, or remedied;
   irreparable; as, an irrecoverable loss, debt, or injury.
   [1913 Webster]

         That which is past is gone and irrecoverable. --Bacon.

   Syn: Irreparable; irretrievable; irremediable; unalterable;
        incurable; hopeless. -- Ir`re*cov"er*a*ble*ness, n. --
        Ir`re*cov"er*a*bly, adv.
        [1913 Webster]

WordNet (r) 3.0 (2006):

irrecoverable
    adj 1: incapable of being recovered or regained [syn:
           unrecoverable, irrecoverable] [ant: recoverable]

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "irrecoverable":
   ago, antiquated, antique, beyond recall, beyond remedy, blown over,
   by, bygone, bypast, cureless, dated, dead, dead and buried,
   deceased, defunct, departed, elapsed, expired, extinct, finished,
   forgotten, gone, gone glimmering, gone-by, has-been, immedicable,
   incorrigible, incurable, inoperable, irreclaimable, irredeemable,
   irreformable, irremediable, irreparable, irretrievable,
   irreversible, irrevocable, lapsed, lost, no more, obsolete, over,
   passe, passed, passed away, past, past hope, past praying for,
   remediless, ruined, run out, terminal, undone, unmitigable,
   unrelievable, unsalvable, unsalvageable, vanished, wound up