It always seemed that the 'Irish question', so long an intractable one, would be the most difficult aspect of Brexit to solve. The Good Friday Agreement was a beautifully crafted piece of political fudge - with enough delicate layers of cross-community power sharing, cross-border cooperation and inter-state dialogue to satisfy everyone - and it's underpinned twenty years of peace in the province.
But when the referendum result bounced the UK Government, unprepared, into defining what Brexit might mean, it adopted a number of seemingly inconsistent positions: first, that there would be no hard border on the island of Ireland, to preserve the peace secured by the GFA; second, that there would be no hard border in the Irish sea, to protect the integrity of the UK; third, that the UK would be leaving 'the' customs union (and, more recently, would not be maintaining 'regulatory alignment' or seeking 'a' customs union). See my discussion of that here: https://www.bdb-law.co.uk/blogs/great-repeal-bill/28-great-repeal-bill-two-three-aint-bad/
The 'sufficient progress' document agreed before Christmas did no more than kick the question a bit further down the road. The EU has become increasingly impatient with the Government's contradictory position, and now (it seems) the crunch has come. Which one of these three positions is the UK Government going to take?
UK negotiators have been warned that the EU draft withdrawal agreement will stipulate that Northern Ireland will, in effect, remain in the customs union and single market after Brexit to avoid a hard border ... [unless] an unexpectedly generous free trade deal, or a hitherto unimagined technological solution, emerge that could be as effective as the status quo in avoiding the need for border infrastructure. As it stands, however, the UK is expected by Brussels to sign off on the text which will see Northern Ireland remain under EU law at the end of the 21-month transition period, wherever it is relevant to the north-south economy, and the requirements of the Good Friday agreement. The move is widely expected to cause ructions within both the Conservative party and between the government and the Democratic Unionist party ...