Green’s Dictionary of Slang

(Aus./Irish) a general excl. of surprise, excitement, alarm etc.

1802		M. & R. Lovell Edgeworth Essays on Irish Bulls 129: ‘By the holy you fleshed ’em,’ says he.
1833		‘Take a Sight’ in Rummy Cove’s Delight in Spedding & Watt (eds) Bawdy Songbooks (2011) III 104: Says he, ‘By the Holy! I’ll just have a row’.
1833		W. Carleton Traits and Stories of Irish Peasantry I 270: By the holy Nelly.
1834		Belfast News Ltr 4 Feb. 4/2: By the holy farmer, Mr Casey, you’re fairly done!
1841		‘Night Before Larry Was Stretched’ in Dublin Comic Songster 186: Oh! by the holy, you thief, / I’ll scuttle your nob with my daddle.
1885		Aberdeen Peoples’ Jrnl 5 Dec. 3/4: ‘By the holy Abraham, I wasn’t guilty’.
1888		W.B. Churchward Blackbirding In The South Pacific 70: By the Holy! I’ll kill the first man who fires a shot.
1900		Bulletin (Sydney) 8 Dec. 28/2: Let the blashted tumbril thieves pick that lock, if they can; an’, by the howly, if I catch them tryin’, I’ll brain ’em wid me collar-chain, so I will!
1911		E. Dyson ‘Barracking’ in Benno and Some of the Push 146: By the holy, you’ll get it agin if you give me too much iv yer gibber.
1927		(con. 1917) J. Stevens Mattock 231: You open up, come clean [...] or by the holy old hell you’ll get coldcocked so your mother won’t know you.