To be fair, "simply" introducing a hard border between parts of your country isn't quite... simply. Which country would do that? In addition, it's a pretty crappy symbol of "getting independence" if you "leave behind" a part of your country to get it, and that's how it's understandably perceived. And May's government relies on the DUP to achieve a majority in parliament, for which this is wholly unacceptable, since it stands for getting closer to the main land and not being one with Ireland. (of course she doesn't have a majority now, but the DUP might very well have broken away long ago if this had been a plan)