Because we hear and see the word irregardless everywhere, we forget that it is not standard English. This is hardly surprising when we consider that irregardless has been in use for at least 100 years in North America.

Most dictionaries list irregardless as non-standard English and advise against its use in speech and writing. The correct word is regardless.

Use regardless

as an adverb meaning "anyway" or "nonetheless"

The deadline seems unrealistic, but we'll get the job done regardless (anyway).

Use regardless of

as a preposition meaning "in spite of"

Joe continued to sing regardless of (in spite of) the jeers and shouts from the audience.

Note: It is unclear how irregardless came about. Some sources suggest that the prefix ir (meaning "not") was combined with the word regard and the suffix less (meaning "without") to create the unnecessary double negative irregardless. Others suggest that irregardless is an unfortunate fusion of the words irrespective and regardless.