Coastie
Someone from the North Coast: Portrush, Portstewart, Coleraine and round about.
Heard in Derry
Yer man's a proper Coastie, he's in the sea at Portrush every mornin' even in January.

Shall I translate it into posh English for you, mucker?
Afternoon! Mind the spaniel. Here it is in posh English.
That gentleman is a true native of the North Coast. He bathes in the sea at Portrush every morning, even in January.
Questions
What does coastie mean in Ireland?
Someone from the North Coast: Portrush, Portstewart, Coleraine and round about.
Where in Ireland do people say coastie?
People say coastie in Derry.
How do you use coastie in a sentence?
“Yer man's a proper Coastie, he's in the sea at Portrush every mornin' even in January.”
Updated
On the list: Irish slang words
Same phrase, other counties
- Derry (not in the dictionary)