WordNet (r) 3.0 (2006):

irregardless
    adv 1: regardless; a combination of irrespective and regardless
           sometimes used humorously

Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "irregardless":
   anyhow, anyway, anywise, at all, at all costs, at any cost,
   at any rate, by any means, despite, even with, however,
   in any case, in any event, in any way, in despite of, in spite of,
   irrespective of, nevertheless, nohow, nonetheless, regardless,
   regardless of, regardless of cost, spite of, with