Green’s Dictionary of Slang

1. a familiar term of address to an unknown male.

1899–1900		C.L. Cullen Tales of the Ex-Tanks 45: Hey, Mike, d’ye want to see the scrap?
1906		‘O. Henry’ ‘The Cop and the Anthem’ in Four Million (1915) 96: ‘Ah there, Bedelia! Don’t you want to come and play in my yard?’ [...] ‘Sure, Mike,’ she said joyfully, ‘if you’ll blow me to a pail of suds.’.
1909		Coshocton (OH) Daily Times 27 Aug. 8/7: Why, hadn’t he lived Here since ’84 and found that the Place was punky? Sure, Mike!
1914		‘Hugh McHugh’ You Should Worry cap. 3: Say, Mike! [...] tip us off to a quiet bunch of eating that will fit a couple of appetites just out seeing the sights.
1936		(con. 1920s) Dos Passos Big Money in USA (1966) 963: Sure, mike, [...] I’ve got enough on him to retire on the blackmail any time now.
1945		A. Kober Parm Me 146: Sure, Mike! [...] A fine listening!
1977		L. Heinemann Close Quarters (1987) 94: Well, Mike, I ain’t gonna slow up on no convoy.

2. a generic term for an Irishman.

1891		J. Maitland Amer. Sl. Dict.
1909		I.L. Nascher Wretches of Povertyville 11: Here Ike and Mike mix jargon and brogue over the bar of a German saloon.
1927		(con. 1918) J. Stevens Mattock 293: Hold yer head up, buddy. Walk straight now, soldier, till old Mike finds ye a flop.
1933		C. Drew ‘Sledgehammer Joe’ in Bulletin (Sydney) 19 July 48/2: ‘The horny-handed stump-diggers will roll up strong [...] to see the local champion in action. They’ll be as thick as Mikes in Ireland’.