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.
"My little sister's soccer team gives every player an MVP trophy at the end of the season regardless of her actual abilities."
How adorable. In this sentence, regardless means without concern, as in every little girl gets a prize without concern for her soccer skills.
"I know you think it's sweet that Cole sends you love notes in Pig Latin and knitted you a sweater using his own hair. Regardless, I think you should break up with him."
In this example, regardless is used to indicate that the speaker isn't concerned with the nice (and kind of weird) things Cole has done. She still thinks her friend should kick Cole to the curb.
"Violetta wears sweatpants everyday regardless of weather or occasion."
We love our sweats as much as the next person, but we don't know how practical this is. Sweatpants at a mid-July pool party? Sweatpants at a funeral? In this totally casual example, regardless means that Violetta doesn't take the forecast or where she's going into consideration before she gets decked out in her favorite sweats.