Irregardless is not a proper English word and should never be used; the correct form is 'regardless.'
While linguists classify 'irregardless' as non-standard, all major English dictionaries (Oxford, Merriam-Webster, Cambridge) list it as an actual word, albeit marked as informal or dialect. The redundant double negative (ir- + -less) likely emerged in American dialects around the early 1900s. Its very existence in widespread use demonstrates how prescriptive rules lose authority when descriptive reality diverges. The term entered Merriam-Webster's Unabridged Dictionary in 1934 and has appeared in published literature for over a century, making it linguistically 'real' even if considered substandard in formal writing.