Irregardless is officially not a word, by the decision of many individuals and organizations who make it their point to tell people what is and is not a word.
Despite the fact the word in question is not a word and does not exist, the definition of the nonword "irregardless" is "in spite of everything; without regard to drawbacks" according to scholars in Princeton University which does exist. The word irrespective is a word which does exist, and according to Princeton, it means exactly the same thing. However, "irrespective" just doesn't have the right ring to it. I feel like a punk kid trying to sound intelligent when I use words like that. Although "irregardless" sounds stupid to a scholar, it sounds much more intelligent to a punk kid, and since I'm little more than a very old punk kid, I use nonwords like "irregardless" and pretend to sound kewl when I really sound like a nitwit.
"Ain't" ain't a word, either. Just in case you're keeping score.