Northern Ireland is part of the United Kingdom, and should not be confused with Ireland, which is a completely separate country. Ballycastle is located in Northern Ireland.
Northern Ireland is not specifically mentioned in canon, although it does field a Quidditch team in the British and Irish Quidditch League - the Ballycastle Bats (QA7).
From the Web
Northern Ireland Tourist Board: Discover Northern Ireland
Wikipedia: Northern Ireland
View 0 thoughts swirling around the pensieve. Add your own.