regardless vs irrespective

regardless is an adjective but irrespective is not an adjective.

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

As adverbs, regardless and irrespective are synonyms defined as:

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

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

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

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