In my humble opinion, it’s not a legitimate one. I’ve heard some great public speakers and a former supervisor say it numerous times. I figure they confused it with irrelevant.
I prefaced this post acknowledging this is my opinion because when I googled the word to grab an accompanying image, I ran across some sites/articles arguing its legitimacy. Yes, while most admit that the word is a double negative because the “less” suffix takes care of the negating factor and the “ir” prefix also denotes something is without regard, they believe that because you know what a person means when they say it, the word is valid. I disagree.
I may know what someone means when they say, “I don’t wish this pain on nobody,” but that doesn’t make the statement any less grammatically incorrect. What’s your take?