The Collaborative International Dictionary of English v.0.48:

Irreligious \Ir`re*li"gious\, a. [L. irreligiosus: cf. F.
   irr['e]ligieux.]
   [1913 Webster]
   1. Destitute of religion; not controlled by religious motives
      or principles; ungodly. Cf. Impious.
      [1913 Webster]

            Shame and reproach are generally the portion of the
            impious and irreligious.              --South.
      [1913 Webster]

   2. Indicating a lack of religion; profane; wicked; as,
      irreligious speech.
      [1913 Webster]

WordNet (r) 3.0 (2006):

irreligious
    adj 1: hostile or indifferent to religion [ant: religious]

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "irreligious":
   amoral, apostate, atheistic, backsliding, blasphemous, creedless,
   disbelieving, faithless, fallen, fallen from grace, godless,
   heretical, impious, incredulous, indevout, irreverent, lapsed,
   minimifidian, nonbelieving, nonreligious, nullifidian, profanatory,
   profane, recidivist, recidivistic, recreant, renegade, repudiative,
   sacrilegious, sinful, un-Christly, unangelic, unbelieving,
   unchristian, unconfident, unconverted, unconvinced, undevout,
   undutiful, ungodly, unholy, unmoral, unreligious, unrighteous,
   unsaintly, wicked, without faith