I keep hearing how "irregardless" is not a word. How can it not be a word? It looks like a word. What is it, if it is not a word?
How do new words get made legitimate?
What people effectively mean when they say something isn't a word is that it's a word that they strongly dislike. As you said, of course it's a word.
The last question is much trickier, though, because it depends on what you mean by "legitimate."
Irregardless is legitimate in the sense that it's a real word that people use. It's even
in dictionaries. Words become legitimate in this broader sense when enough people use them.
But if by "legitimate" you mean standard, then that's something different.
Irregardless is a real word, but it's nonstandard because it's not accepted by enough educated speakers. And that's unlikely to happen with
irregardless because of its ill-formedness and its frequent use by less-educated speakers.
