Green’s Dictionary of Slang

(Irish) a small glass of whisky.

1908		L. Doyle Ballygullion 74: All that day he kept thinkin’ about it, an’ dhrinkin’ half-wans av whiskey to dhrive the thoughts out av his head.
1922		Joyce Ulysses 306: What’ll it be, Ned? – Half one, says Ned.
1936		Irish Times 20 March n.p.: If help is near at hand ‘a half one’ – in the vulgar vernacular – will be of benefit [BS].
1946		D. MacDonagh Happy as Larry Act IV: The half-ones ordered of poisonous stuff / And then the half-jarred garrulous guff.
1953		Lynn Doyle Back to Ballygullion 76: Make it three half-ones, Tammas.
1961		(con. 1890–1910) ‘Flann O’Brien’ Hard Life (1962) 176: He led the way into a public house in Suffolk Street and to my surprise agreed to drink half-ones instead of balls of malt.
1962		B. Behan Brendan Behan’s Island (1984) 105: chuckles: [...] Sixteen half ones of malt and chasers. barman: Certainly, Chuckles ... (Shouts) sixteen small whiskeys and sixteen bottles of stout.
1970		C. Brown Down All the Days 4: Their father had been drinking since he was twelve, when a pint of stout cost only a penny and a ‘half one’ three-halfpence.
1983		W.F Marshall ‘The Runaway’ in Livin’ in Drumlister 76: Sez I, ‘Wull ye come for a half-wan?’.
1993		(con. 1930s) M. Verdon Shawlies, Echo Boys, the Marsh and the Lanes 35: He took her for a jorum. She started on whackers, but changed to meejums. He was on half ones.