Only for a short period. The UK government are introducing an ESTA like system for non-Irish visitors from the EU, so legally if you're not an Irish citizen you

Only for a short period. The UK government are introducing an ESTA like system for non-Irish visitors from the EU, so legally if you're not an Irish citizen you should apply for this to cross the border. I mean, I don't expect this to be enforced for most travellers but I suspect if you want to stay in NI then you'll probably need to show a visa (based on current UK law).