CGP Grey had a good video about this recently: [0].
Basically, the UK is required by treaty with RoI to not impose a border between the RoI (not part of the UK) and Northern Ireland (part of the UK). And the UK wants to erect a border between the UK and the EU. So, do they break the treaty with RoI potentially causing The Troubles [1] to come back? Or does Northern Ireland effectively leave the UK? Or does the UK allow the EU to require that Northern Ireland and the UK by extension to conform to the trade rules the EU made and will make?
They can only pick one, and they don't want to pick any of them.
The UK doesn't need a wall -- it could have an open border if it wants to (right?). It's Ireland that would have to enforce a border. Unless there is some other binding agreement at play.
It absolutely could... but a major promise of the Brexit campaign was to "secure their borders" so that's a no-go. Plus it'd result in a hell of a lot of smuggling into the UK, and they'd be sanctioned under WTO rules since they'd be "favouring" Irish products and discriminating against others. For example US apples that came into a British port would have to pay tariffs while Irish apples driven across the border wouldn't, which is a big no-no under WTO rules where the first and foremost rule is that you must treat countries the same using WTO rules. So they'd be forced to either drop checks everywhere, or to erect them at the Irish border, or to do this legally they'd need a separate Trade Deal with Ireland, that is with the EU... which puts them right back at square one.
Or they pick the third option. They keep the northern Irish open, and simply deport all non Irish without proper documentation.
If course the EU won't allow a backdoor into the common market, so they will be the ones pressuring Ireland for a hard border. Whatever hard border would be imposed solely by the Irish republic, which means the British government abides by the good Friday agreement.
Basically, the UK is required by treaty with RoI to not impose a border between the RoI (not part of the UK) and Northern Ireland (part of the UK). And the UK wants to erect a border between the UK and the EU. So, do they break the treaty with RoI potentially causing The Troubles [1] to come back? Or does Northern Ireland effectively leave the UK? Or does the UK allow the EU to require that Northern Ireland and the UK by extension to conform to the trade rules the EU made and will make?
They can only pick one, and they don't want to pick any of them.
[0] https://www.youtube.com/watch?v=J1Yv24cM2os [1] https://en.wikipedia.org/wiki/The_Troubles