What the GFA does state is that 'the present wish of a majority of the people of Northern Ireland, freely exercised and legitimate, is to maintain the Union and, accordingly, that Northern Ireland's status as part of the United Kingdom reflects and relies upon that wish; and that it would be wrong to make any change in the status of Northern Ireland save with the consent of a majority of its people'.
Remind the audience what the majority of Northern Ireland’s people decided with regards to EU membership. Did that constitute consent to have Northern Ireland’s existing status as part of an EU member state changed?
The UK government talked itself into a situation where it had three demands that it could not back down on: avoid a hard border, avoid special treatment of NI vis-a-vis the rest of the UK, and leave the EU customs union. It doesn’t take a genius to see that these are contradictory.
The border down the Irish sea is the best solution, pragmatically, as there are only a handful of points of entry, and the Irish customs and revenue are already prepared to facilitate the considerable portion of NI’s imports from the rest of UK that landing at Dublin or Rosslare for onward transport into Northern Ireland (look at a road-map to see why this happens). This makes customs compliance a bulk issue that can be started before crossing the sea, not a check on every truck, van or trailer as it hits a road checkpoint.
Farming is the most affected industry: many farmers on one side of the border sell their produce to a processing company on the other; some food products cross the border multiple times: milk from Southern cows goes to a Northern dairy, is pasteurised, converted to whey and sold to a Southern cheese factory which sends the bulk cheese to Northern Ireland to be packaged for retail; some of that final product then goes to GB, and some comes back South again. Put a border there, and those businesses will close, and NI is not exactly a booming economy as it is.
The alternative to a sea border is to erect customs posts at the 275 different places where a road crosses the border (and you can come up with some fun suggestions for the 11 roads that actually
form the border, with one side in the Republic and the other in Northern Ireland), or just decide to do nothing and brace yourself for a wave of smuggling: something that’s already a problem. Smuggling and other organised crime is already a major funding source for paramilitaries (although it’s closer to the truth to say that paramilitarism is a major cover for organised crime in the North).