Among the problems thrown up by the UK exiting the EU, the most intractable (and yet most predictable) may be the UK's only land border: the one between Northern Ireland and the Republic of Ireland.

The 310 mile (500 km) border has more than 200 land crossings, some of which are laneways through someone's farm, and was impossible to monitor even at the height of "The Troubles". Today, you can cross it without noticing, except when kilometre signs turn to mileage and the phone signal switches to a different provider. Thousands of people live and work on opposite sides, or cross it for other reasons, including medical treatment. This is made easier by both jurisdictions being within the EU. The Good Friday Agreement, which brought peace to 1998 after years of negotiation and compromise, assumes that RoI and the UK will work together "as friendly neighbours and as partners in the European Union".

Brexit complicates that situation, to say the least.

Because Britain is on course to leave the E.U.ís tariff-free single market, imports and exports must somehow be monitored across this 310-mile-long barrier....
So how can some system be worked out? The U.K. remaining in the single market is an obvious solution, but thatís not on the table. Another possibility: keeping Northern Ireland in the single market, along with the Republic, and having a customs border along the Irish Sea....

The main obstacle to this solution, however, would be the above-mentioned DUP, who are suspicious that a Northern Irish customs union with the Republic of Ireland but not with the U.K. might be a creeping move toward the pan-Irish unification they are constitutionally opposed to. Given that the DUP is currently keeping Theresa Mayís government in power, it holds serious sway.....
If a solution isnít worked out, Brexit negotiations may stall, raising the risk of Britain crashing out of the E.U. with no deal. Some sort of compromise could ultimately be worked out, but current British approaches donít inspire much confidence: So far, the problem is being greeted with a mix of magical thinking and shouting.
https://www.citylab.com/equity/2017/...border/546887/

What is particularly frustrating is that anyone who knew anything about NI could have predicted this. The issue was ignored and kicked down the line, and now Theresa May's govt is claiming surprise. It would be farcical if the stakes weren't so high.