Looking through the Dental rule book, it only talks about treatment in Ireland but it doesn't explicitly state if that's the Republic of Ireland or the island of Ireland.
That said, the website of DeCare Dental Ireland, the company that pays the claims (VHI market and sell the product on behalf of DeCare Dental), says that any registered dentists in the Republic or Northern Ireland is recognised - [broken link removed].
If you haven't done so already, it's probably best to give VHI a call to determine if you're covered or not. In fact, I'd recommend doing this before any treatment with any insurer, just so there are no surprises.
PS You might get more (and possibly better!) responses by moving this thread to the Insurance Sub forum. Just use the Thread Tools button just above your first post.