irrespective vs regardless

irrespective is not an adjective while regardless is an adjective.

Word	Noun	Adjective	Verb	Adverb
irrespective	No	No	No	Yes
regardless	No	Yes	No	Yes

As adverbs, irrespective and regardless are synonyms defined as:

irrespective and regardless: in spite of everything; without regard to drawbacks

Other synonyms of irrespective include disregarding, disregardless, no matter.

irrespective (adjective)	regardless (adjective)
	(usually followed by `of') without due thought or consideration

irrespective (adverb)	regardless (adverb)
in spite of everything; without regard to drawbacks	in spite of everything; without regard to drawbacks