Swinging Jesus
An exclamation of alarm or disbelief. Roughly: oh bloody hell.
Swingin' Jaysus, who left de gate open? De cows are halfway to Mullingar!

Shall I translate it into posh English for you, head?
I say, what a pleasant surprise. Here it is in posh English.
Merciful heavens, who neglected to close the gate? The cattle are halfway to Mullingar!
Questions
What does swinging Jesus mean in Ireland?
An exclamation of alarm or disbelief. Roughly: oh bloody hell.
How do you use swinging Jesus in a sentence?
“Swingin' Jaysus, who left de gate open? De cows are halfway to Mullingar!”
Updated
On the list: Irish slang phrases