Lookups spiked on February 1, 2012.
A much-forwarded list of "Top 10 Grammar Peeves" included the warning, "Irregardless is not a word," which inspired many people to look it up.
As they discovered, it is a word - although not one worth using.
Irregardless probably emerged as a confused blend of the synonyms irrespective and regardless. It does appear in print, but is more often used in speech, and its entry in the dictionary contains the following advice: "Use regardless instead."