For the past 20 years, the border between Northern Ireland - a part of the United Kingdom - and the Republic of Ireland has been open.
Both sides have benefitted economically as members of the European Union, but there are fears that a no-deal Brexit might turn back the clock.
Al Jazeera's Neave Barker reports from Carlingford Lough on the border between the two countries, which could soon be subject to a new territorial dispute.
