Regardless of what you may read or hear people say, irregardless is an unnecessary and illogical substitution for regardless.
For some strange reason, this hideous word has slipped into the English language.Think for a second about the logic behind the construction of the word irregardless: The prefix ir- means “the absence of,” and the suffix -less means “the absence of.” Is it really necessary to indicate the absence of regard twice? I think not.
Remember that regardless is a complete word on its own without the redundant ir- at the beginning.
My opinion is that people are confused between “regardless” and “irrespective” (which is the word they are probably looking for) and they compromise by mashing them together into “irregardless”. At least this error helps to provide a warning that what they are saying is probably suspect.