I had a relative who used to say irregardless every time she should have been saying regardless, but because I was a child, I knew that I wasn’t allowed to correct her. Well, that was a long time ago, and I can now sum up in one short sentence what I was thinking every single time I heard this grammar error: Irregardless isn’t a word; the word is regardless.
By saying irregardless, you’re saying the opposite of what you really mean; regardless already means despite or without regard, and the prefix –ir is a negative, so irregardless would mean not despite or not without regard, which is completely illogical.
Here’s an example: I’m going to quit my job irregardless of the consequences. This means I’m going to quit without not having any regard for the consequences or not despite the consequences.
What you want to say is I’m going to quit my job regardless of the consequences, which means I’m going to quit without any regard for the consequences or despite the consequences.
I see that irregardless is in more modern dictionaries as non-standard (it should be in there as substandard), and one online dictionary says that it is used instead of the correct word, regardless, in casual speech and writing and sometimes even by more educated people even though it’s considered to be a grammar faux-pas. That is a perfect example of what I’ve said on many occasions and what Paul has said on many occasions: Just because it’s in the dictionary doesn’t mean that it’s correct, and just because someone who is considered to be intelligent uses it doesn’t mean that it’s correct. You have to look beyond that to decipher what’s really going on.
As I like to say, irregardless is not a word regardless of its presence in the dictionary—period.