The rules for using regardless and irregardless are easy.
You use regardless as an adverb when you want to say anyway or without regard or concern, as in I have no idea how many people are coming to the party, but I need to buy snacks regardless.
You use irregardless… never. Because it's not a word.
Nobody knows for sure where this made-up term comes from, but many speculate that it's a mash-up of the words irrespective and regardless. The problem is, when you blend those two words and get irregardless, you get a double negative.
Check it out: the prefix –ir means not. The suffix –less means without. So the faux word irregardless means not without regards, which means the same thing as with regards.
Confused? You should be. Because irregardless is not a word.
So don't use it.