You already accepted being split, because its obvious that you never unified. That ship sailed long ago. Even the NI aren't unified. The ROI is a foreign country to the UK (as you wanted. you won that fight.) and a border, invisible or not, has been there for ages. Every soverign nation on earth has the right to control who enters and leaves. This is no different. Did you think you could be independent and still get a say in our internal political decisions? Why shouldnt you have to show your passport to enter the UK? Because a few foreign rabblerousers want to play terrorist? Every other group will have to do so post brexit. If scotland gains independence, then they will have the option to put a border there too.