Irregardless is a century-old colloquial word that means the same as irrespective and regardless, and it may have come about by some fusion or confusion of those two words. The use of irregardless is a common peeve among people who question illogical new words and phrases in English, but the word is not as bad as many people think.
The main gripe is that irregardless is an illogical word because it contains a double negative. The prefix ir- means not and the suffix –less mean without, so the word literally means not without regard, which is the opposite of its intended meaning. Plus, irregardless can be annoying because it often takes the place of the perfectly good regardless and irrespective, which don’t deserve to be pushed out of the language by a logic-flouting upstart like irregardless.
But serious usage problems are those that create ambiguity or that erode the meaning of long-established, useful words. No English speaker who hears the word irregardless actually interprets it as meaning not without regard. We might find the word annoying, but we know exactly what the speaker means. And though irrespective and regardless have perhaps lost some ground since the arrival of irregardless, they are still widely used, and they prevail over irregardless by a large margin in edited publications.
This is not to say that irregardless is a good word. It is probably best avoided by careful writers, especially in any type of formal writing, if only because so many people despise it. It certainly has no place in school papers or work documents. Still, there are better language peeves to have than this mostly harmless colloquialism.
Just for fun, here are a few examples of irregardless apparently used in earnest:
Perhaps, as has been suggested, patients get better irregardless of antibiotics, and not because of them. [Patch]
Her instruction allows them to create a work of art irregardless of their ability. [Towanda Daily Review]
[T]he underlying point is the dictation of one’s destiny for the benefit of another, irregardless of whether or not such exploitation results in a thriving new life in Georgia. [GlobalResearch.ca]
There are compelling reasons why President Obama is the favorite in this fall’s election irregardless of which candidate gets the GOP nod. [East Valley Tribune]
We tried to find examples in the edited publications that we usually check, but there are no instances of irregardless to be found in these sources, while there are many instances of irrespective and regardless. So the logical, established words are doing fine.