Irregardless is an informal term commonly used in place of regardless or irrespective. It has caused controversy since it first appeared in the early twentieth century and most dictionaries list it as “nonstandard” or “incorrect”. The origin of the word is not exactly known but is said in some reference books to be a portmanteau word, of irrespective and regardless, both of which are accepted standard English words.
According to the Oxford English Dictionary, Irregardless was first acknowledged in 1912 by the Wentworth American Dialect Dictionary as originating from western Indiana. In 1923, Literary Digest published an article titled “Is There Such a Word as Irregardless in the English Language?”
Merriam Webster Dictionary has this to say about the “word”: “Irregardless originated in dialectal American speech in the early 20th century. Its fairly widespread use in speech called it to the attention of usage commentators as early as 1927. The most frequently repeated remark about it is that “there is no such word.” There is such a word, however. It is still used primarily in speech, although it can be found from time to time in edited prose. Its reputation has not risen over the years, and it is still a long way from general acceptance. Use regardless instead.“ (wiki)