Green’s Dictionary of Slang

(Irish) a general excl. of amazement, disbelief.

1908		L. Doyle Ballygullion 123: ‘Boys-a-boys,’ sez James Dougherty, ‘the works av man is wonderful.’.
1943		(con. 1850–60s) G.A. Little Malachi Horan Remembers 30: Every townland had on everything a different way of going and a different way of living. — Boys-o-boys — boys-o-boys.
1966		P. Boyle At Night All Cats are Grey 156: Boys-a-boys, thon’s a queer looking yoke.
1979		J. Morrow Confessions of Proinsias O’Toole 98: Boys-a-boys! That’s one quare shooter, lads.
2002		O’Byrne Files: Dublin Sl. Dict. 🌐 Boys o boys phr. Grunted sigh used to punctuate speech as an expression of tiredness.