Irregardless of the literary backlash that it evokes in some circles, the word has been used for quite a long time, the first time it appeared in print was way back in 1795. People have had ample opportunity to be offended by it for a couple of centuries now.
The adverb is thought to be a blend of regardless and irrespective. Now snooty dictionaries still look down their pince nez clad collective noses at it, even calling it non standard, but they do admit it is a word, albeit a rather low brow one.
Some of the derision that is heaped upon the word derives from the fact that regardless already means without regard, so irregardless is in fact a double negative. But the word has a clear meaning and has not been struck by the dictionaries, who merely hold their nose and retort that the users of the word tend to lack education and refinement.
Here's what my old friend Merriam Webster has to say about it:
Irregardless was popularized in dialectal American speech in the early 20th century. Its increasingly widespread spoken use called it to the attention of usage commentators as early as 1927. The most frequently repeated remark about it is that "there is no such word." There is such a word, however. It is still used primarily in speech, although it can be found from time to time in edited prose. Its reputation has not risen over the years, and it is still a long way from general acceptance. Use regardless instead.Now the Chicago Manual does say it is a word but discourages its use because it raises genteel people's hackles so. So I am ill bred. Sue me.