Location: County Antrim, Northern Ireland
Ballycastle, Irish Gaelic Baile an Chaistil ("town of the castle") is located in County Antrim, Northern Ireland. It is part of the United Kingdom. The game of Quidditch is mentioned on its Wikipedia page.
Lexicon map: Quidditch in Britain
From the Web
Writing by J K Rowling on Pottermore: The Daily Prophet
Harry Potter Wiki: Ballycastle