As an atheist, what angers you?

[Edwardo Piet]

It's become an informal version of "regardless" through bastardization. Sadly it has indeed become an actual word.

I say "sadly" because it annoys me how it makes no sense whatsoever. How can "regardless" and "irregardless" mean the same thing?

I think language should change so both "rational" and "irrational" to mean "rational" and if you actually want to call someone "irrational" you have to say "rationalless" or "irrationalless" which both mean "irrational".