Regardless means “without regard to; in spite of (something).”

Irrespective means “heedless; characterised by disregard.”

Practically interchangeable, but you get to choose only one of these two words…you can’t use bits from both. I don’t care what some web site says about “non-standard use”; that is simply a polite way to say “illiterate use”, and IRREGARDLESS IS NOT A GODDAMNED WORD! EVERY TIME YOU USE IT, GOD KILLS A KITTEN AND BABY JESUS CRIES. JUST STOP-STOP-STOP!