The Collaborative International Dictionary of English v.0.48:

Irreparable \Ir*rep"a*ra*ble\, a. [L. irreparabilis: cf. F.
   irr['e]parable. See In- not, and Reparable.]
   Not reparable; not capable of being repaired, recovered,
   regained, or remedied; irretrievable; irremediable; as, an
   irreparable breach; an irreparable loss. --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

irreparable
    adj 1: impossible to repair, rectify, or amend; "irreparable
           harm"; "an irreparable mistake"; "irreparable damages"
           [ant: rectifiable, reparable]

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "irreparable":
   beyond recall, beyond remedy, cureless, gone, immedicable,
   impossible, incorrigible, incurable, inoperable, irreclaimable,
   irrecoverable, irredeemable, irreformable, irremediable,
   irretrievable, irreversible, irrevocable, lost, past hope,
   past praying for, remediless, ruined, terminal, uncorrectable,
   undone, unmitigable, unrelievable, unsalvable, unsalvageable