The Collaborative International Dictionary of English v.0.48:

Irremediable \Ir`re*me"di*a*ble\, a. [L. irremediabilis: cf. F.
   irr['e]m['e]diable. See In- not, and Remediable.]
   Not to be remedied, corrected, or redressed; incurable; as,
   an irremediable disease or evil.
   [1913 Webster]

WordNet (r) 3.0 (2006):

irremediable
    adj 1: impossible to remedy or correct or redress; "an
           irremediable error"; "irremediable defects of character"
           [ant: remediable]

Moby Thesaurus II by Grady Ward, 1.0:

51 Moby Thesaurus words for "irremediable":
   ausgespielt, bankrupt, beyond recall, beyond remedy, blasted,
   blighted, broken, cureless, desolated, destroyed, devastated,
   done for, done in, down-and-out, fallen, finished, gone,
   gone to pot, immedicable, impossible, in ruins, incorrigible,
   incurable, inoperable, irreclaimable, irrecoverable, irredeemable,
   irreformable, irreparable, irretrievable, irreversible,
   irrevocable, kaput, lost, overthrown, past hope, past praying for,
   ravaged, remediless, ruined, ruinous, spoiled, terminal,
   uncorrectable, undone, unmitigable, unrelievable, unsalvable,
   unsalvageable, wasted, wrecked