I heard it proposed by Michael Portillo (BBC This Week 13/6/19), that a Johnson government might solve the Irish conundrum by holding a referendum in Northern Ireland, which voted by a majority to Remain in 2016.
If the vote was similar, Northern Ireland would remain in the EU, or at least stay in the Customs Union and the border could then stay open. Edit I do not believe it is envisaged that this would involve Irish unification.
However that would involve the goods entering the mainland UK being controlled.
So what puzzles me is how Johnson would expect to get the agreement of the DUP, who would surely oppose it - as they have continually done any idea of a "border in the Irish sea".
And without DUP support how could the Johnson Tories get an Act passed for a Northern Ireland Referendum?
Another problem to which it would give rise would be that if Northern Ireland, as part of the UK, remained in the EU the SNP would have an even stronger case for Scotland remaining. So would we have to rebuild Hadrian's Wall, not to keep out the Scottish Barbarians, as the Romans found necessary, but their duty-free goods?