Regardless is an adverb that means without paying attention to the situation of circumstances.
“Susan and Matt kept driving regardless of the pounding rain.”
Nowadays, you will hear many people say ‘irregardless’. This is not grammatically correct, but may have come into play because the prefix ‘ir’ is used to mean ‘not’ – think ‘responsible/irresponsible’.
Even though regardless already has that sense of ‘not paying attention’, some people emphasize the ‘not’ by saying irregardless.