Back pocket in a vest
Back pocket in a vest is Irish slang for totally useless. No use to man nor beast.
Sure that's about as much use as a back pocket in a vesht!

Do come and sit by the fire. Would you like me to translate 'Back pocket in a vest' in use for you?
I must say, that is roughly as useful as a hip pocket sewn into an undershirt.
Any good?
Questions
What does back pocket in a vest mean in Ireland?
Totally useless. No use to man nor beast.
How do you use back pocket in a vest in a sentence?
“Sure that's about as much use as a back pocket in a vesht!”
Updated