The Collaborative International Dictionary of English v.0.48:

Irreproachable \Ir`re*proach"a*ble\, a. [Pref. ir- not +
   reproachable: cf. F. irr['e]prochable.]
   Not reproachable; above reproach; not deserving reproach;
   blameless.
   [1913 Webster]

         He [Berkely] erred, -- and who is free from error? --
         but his intentions were irreproachable.  --Beattie.
   [1913 Webster]

WordNet (r) 3.0 (2006):

irreproachable
    adj 1: free of guilt; not subject to blame; "has lived a
           blameless life"; "of irreproachable character"; "an
           unimpeachable reputation" [syn: blameless,
           inculpable, irreproachable, unimpeachable]

Moby Thesaurus II by Grady Ward, 1.0:

93 Moby Thesaurus words for "irreproachable":
   Christian, above suspicion, absolute, beyond all praise, blameless,
   chaste, clean, creditable, decent, defectless, erect, errorless,
   estimable, ethical, exemplary, exquisite, fair, faultless,
   flawless, full of integrity, good, guiltless, high-minded,
   high-principled, highly respectable, honest, honorable, ideal,
   immaculate, impeccable, inculpable, indefectible, indefective,
   infallible, innocent, inviolate, irreprehensible, irreprovable,
   just, just right, law-abiding, law-loving, law-revering, manly,
   moral, noble, peerless, perfect, principled, pure, reputable,
   respectable, right, right-minded, righteous, sinless, spotless,
   stainless, sterling, taintless, true-dealing, true-devoted,
   true-disposing, true-souled, true-spirited, truehearted,
   unadulterated, unarraignable, unblamable, unblameworthy,
   unblemished, uncensurable, uncontaminated, uncorrupt, uncorrupted,
   undefiled, unexceptionable, unfaultable, unimpeachable,
   unindictable, unmixed, unobjectionable, unspotted, unstained,
   unsullied, untainted, untarnished, upright, uprighteous,
   upstanding, virtuous, worthy, yeomanly