Your head's cut
You're mad, you're talking nonsense. Said with a shake of the head.
Ye want to walk to Bangor in dem shoes? Yer head's cut.

Shall I translate it into posh English for you, mucker?
Do come and sit by the fire. Here it is in posh English.
You propose to walk to Bangor in those shoes? You have taken leave of your senses.
Questions
What does your head's cut mean in Ireland?
You're mad, you're talking nonsense. Said with a shake of the head.
How do you use your head's cut in a sentence?
“Ye want to walk to Bangor in dem shoes? Yer head's cut.”
Updated
On the list: Irish slang phrases