This adverb, apparently a blend of irrespective and regardless, originated in dialectal American speech in the early 20th century (according to the American Dialect Dictionary, it was first recorded in western Indiana in 1912). Its use in nonstandard speech had become widespread enough by the 1920s to make it a natural in a story by Ring Lardner . . . [and] also a natural in books by usage commentators. The most frequently repeated comment about irregardless is that "there is no such word.

Word or not, irregardless has continued in fairly common spoken use, although its bad reputation has not improved with the years. It does occur in the casual speech and writing of educated people, and it even sometimes finds its way into educated prose. But irregardless is still a long way from winning general acceptance as a standard English word. Use regardless instead.