Northern Ireland is part of the UK, but because of a special power-sharing agreement, it has an open border with Republic of Ireland. This was designed as a compromise that ended 30 years of conflict and violence in Northern Ireland between Nationalist and Unionist paramilitaries. Today, Brexit means that the UK needs to close its borders and the issue of the Irish border is one of the hardest things to negotiate with the EU. Closing this border could undermine the compromise that kept the peace for 20 years.