Yes. It may not be a word that you like, or a word that you would use in a term paper, but irregardless certainly is a word. It has been in use for almost 200 years, and is employed by a large number of people across a wide geographic range and with a consistent meaning. That is why we, and well-nigh every other dictionary of modern English, define this word. Remember that a definition is not an endorsement of a word’s use.
Does irregardless mean the same thing as regardless?
Yes. We define irregardless as "regardless." Many people find irregardless to be a nonsensical word, as the ir- prefix usually functions to indicate negation; however, in this case it appears to function as an intensifier. Similar ir- words, while rare, do exist in English, including irremediless ("remediless"), irresistless ("resistless"), and irrelentlessly ("relentlessly").
Is irregardless slang?
We label irregardless as “nonstandard” rather than “slang.” When a word is nonstandard it means it is “not conforming in pronunciation, grammatical construction, idiom, or word choice to the usage generally characteristic of educated native speakers of a language.” Irregardless is a long way from winning general acceptance as a standard English word. For that reason, it is best to use regardless instead.