Regardless is a word in standard English. Irregardless is not.
Short version of the story: the less takes care of having to put ir in front of regardless.
Don’t say “irregardless.” Just say “regardless.”
(Apparently, irregardless is a word in some dialects, used to shut down a conversation. So unless you grew up with it as part of your culture, and you want to be rudely abrupt, don’t use it!)