The Collaborative International Dictionary of English v.0.48:

Irreligion \Ir`re*li"gion\, n. [L. irreligio: cf. F.
   irr['e]ligion. See In- not, and Religion.]
   The state of being irreligious; lack of religion; impiety.
   [1913 Webster]

WordNet (r) 3.0 (2006):

irreligion
    n 1: the quality of not being devout [syn: irreligiousness,
         irreligion]

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "irreligion":
   apostasy, atheism, backsliding, desertion, fall from grace,
   godlessness, impiety, impiousness, irreverence, lapse,
   lapse from grace, recidivism, recreancy, sinfulness,
   un-Christliness, unangelicalness, unchristianliness, undutifulness,
   ungodliness, unholiness, unrighteousness, unsaintliness,
   wickedness



The Devil's Dictionary (1881-1906):

IRRELIGION, n.  The principal one of the great faiths of the world.