Green’s Dictionary of Slang

ganeys n.

In exclamations

by ganeys! (also by gannies!) [janey mack! excl.]

a general excl.

1950		L. O’Flaherty Insurrection 156: By Ganeys! That looks like the king of all guns.
1954		Dublin Mag. Oct/Dec. 32: Let the wife say what she likes, by gannies, I’l go and have a couple of good balls of malt for myself .