As of right now, Britain is part of the European Union, but it is expected to leave it soon. This would have consequences in Northern Ireland which is part of the united kingdom. A separation would mean for them that they would need a real border separating Northern Ireland from Ireland. Which would mean that they couldn't rely as much anymore on them on aspects such as economical and agricultural. Also imposing a border would be very difficult and costly since about 300 roads are crossing the border.