The Collaborative International Dictionary of English v.0.48:

Irreverence \Ir*rev"er*ence\, n. [L. irreverentia: cf. F.
   irr['e]v['e]rence.]
   The state or quality of being irreverent; lack of proper
   reverence; disregard of the authority and character of a
   superior.
   [1913 Webster]

WordNet (r) 3.0 (2006):

irreverence
    n 1: an irreverent mental attitude [ant: reverence]
    2: a disrespectful act [syn: irreverence, violation]

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "irreverence":
   apostasy, atheism, backsliding, desertion, discourtesy, disesteem,
   dishonor, disparagement, disrespect, disrespectfulness,
   fall from grace, impiety, impiousness, impudence, insolence,
   irreligion, lack of respect, lapse, lapse from grace, recidivism,
   recreancy, ridicule, undutifulness