The OED on "irregardless"

irregardless, a. and adv.

Chiefly N. Amer.

In non-standard or humorous use: regardless.

1912 in Wentworth Amer. Dial. Dict.
1923 Lit. Digest 17 Feb. 76 Is there such a word as irregardless in the English language?
1934 in Webster (labelled Erron. or Humorous, U.S.).
1938 I. KUHN Assigned to Adventure xxx. 310, I made a grand entrance and suffered immediate and complete obliteration, except on the pay-roll, which functioned automatically to present me with a three-figure cheque every week, ‘irregardless’, as Hollywood says.
1939 C. MORLEY Kitty Foyle xxvii. 267 But she can take things in her stride, irregardless what's happened.
1955 Publ. Amer. Dial. Soc. XXIV. 19, I don't think like other people do and irregardless of how much or how little dope would cost me [etc.].
1970 Current Trends in Linguistics X. 590 She tells the pastor that he should please quit using the word ‘irregardless’ in his sermons as there is no such word.
1971 M. MCSHANE Man who left Well Enough iv. 96 The sun poured down on Purity irregardless of the fact that it received no welcome.