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: 

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?