Merriam-Webster's Collegiate dictionary (http://www.m-w.com/) has this to say:
"Etymology: probably blend of irrespective and regardless
"Date: circa 1912
"Irregardless originated in dialectal American speech in the early 20th century. Its fairly widespread use in speech called it to the attention of usage commentators as early as 1927.
"The most frequently repeated remark
about it is that "there is no such word." There is such a word, however. It is still used primarily in speech, although it can be found from time to time in edited prose. Its reputation has not risen over the years, and it is still a long way from general acceptance. Use regardless instead."
Sorry for the cut-and-paste there, but I think M-W has hit the nail right on the head. I'm tired of hearing people say that "irregardless" is not a word. Irregardless is a word. It is non-standard, and not a particularly good word, and should not be used, especially since there are better alternatives (ie "regardless"). Besides, you might get poked with a big pointy stick -- repeatedly.
However, and this bears repeating: Irregardless is a word.
Search irregardless on dictionary.com for why the word is not exactly non-standard as well.