The UK doesn’t leave the Customs Union. By far the simplest. But the demented ideologues of Brexit see leaving the CU as the talismanic, totemic, ne plus ultra of Brexit. So that is a non starter
A full hard militarised India-Pakistan, West Bank type border. Not going to happen.
Ireland leaves the EU. Err no, thats a non starter.
A united Ireland. Definitionally no UK EU land border in that case. This is a long way off so wont happen if at all before March 2019. So that is a non starter
NI as part of the CU, but rUK not. Politically that would be a non starter for the DUP who are the tail that wags the westminster dog so that is off. Economically most NI trade is with the UK so thats that dead in the water.
So some form of increased hardness over the existing border is going to happen.
This can’t be done with electronic surveillance and IT alone. Leave aside the technical, legal and other challenges.
A return to the days of spiked roads and hard customs posts would be a gift without parallel to the still active dissident republicans. So thats a problem
Anything in between would allow for an increase in the massive smuggling that already goes on. Where now it is confined to fuel , this would expand to Every. Single. Thing. Presumably Brexit doesn’t mean “lets give more cash to ex IRA and UVF hoods who now engage in smuggling” .
So expect a border zone of control, where there is much increased mobile customs and immigration, much more intrusive and engaged than now, increased disruption to the daily lives of those on and near the border. Thats going be popular.