The approach taken by lexicographers when documenting a word’s uses and limitations can be prescriptive or descriptive. The method used with irregardless is overwhelmingly prescriptive.
I try not to be a huge grammar weeny at work, but I broke down yesterday when someone used “Irregardless.”
It turns out to be a hotly contested issue in linguistics, and if you believe it’s a lexicographer’s job to describe a language, you allow the silly word, and if you believe it’s a lexicographer’s job to detail how a language should be spoken, then I was right.
According to Wikipedia, though, “ain’t” is a different problem altogether.
