Grab - Irregardless of wot yew fink, yes it is a word. According to the OED it's early 20thC, no indication that it's Yank (I've heard & seen it used in the UK, & used it myself), & they reckon it's an informal combination of irrespective & regardless. Personally, I've always regarded the "ir-" as an intensifier & use it when I particularly want to emphasize the 'regardless' bit. Which seems to sit pretty well with the OED combo origin!