The Collaborative International Dictionary of English v.0.48:

Irreverent \Ir*rev"er*ent\, a. [L. irreverens, -entis: cf. F.
   irr['e]v['e]rent. See In- not, and Reverent.]
   Not reverent; showing a lack of reverence; expressive of a
   lack of veneration; as, an irreverent babbler; an irreverent
   jest.
   [1913 Webster]

WordNet (r) 3.0 (2006):

irreverent
    adj 1: showing lack of due respect or veneration; "irreverent
           scholars mocking sacred things"; "noisy irreverent
           tourists" [ant: reverent]
    2: characterized by a lightly pert and exuberant quality; "a
       certain irreverent gaiety and ease of manner" [syn:
       impertinent, irreverent, pert, saucy]
    3: not revering god [syn: godless, irreverent]

Moby Thesaurus II by Grady Ward, 1.0:

37 Moby Thesaurus words for "irreverent":
   apostate, atheistic, aweless, backsliding, blasphemous, cheeky,
   derisive, discourteous, disparaging, disrespectful, fallen,
   fallen from grace, flip, flippant, impertinent, impious, impudent,
   insolent, insulting, irreligious, lapsed, mocking, profanatory,
   profane, recidivist, recidivistic, recreant, renegade, ridiculing,
   rude, sacrilegious, saucy, uncivil, undutiful, ungodly, unhallowed,
   unholy