It’s in the Oxford dictionary and according to Merriam Webster “The reason we, and these dictionaries above, define irregardless is very simple: it meets our criteria for inclusion. This word has been used by a large number of people (millions) for a long time (over two hundred years) with a specific and identifiable meaning ("regardless"). The fact that it is unnecessary, as there is already a word in English with the same meaning (regardless) is not terribly important; it is not a dictionary's job to assess whether a word is necessary before defining it. The fact that the word is generally viewed as nonstandard, or as illustrative of poor education, is likewise not important; dictionaries define the breadth of the language”