It was always there for all to see, the great Celtic stone cross barring the way to Brexit. Finally, as crunch day nears, the government and its Brextremists have to confront what was always a roadblock to their fantasies. They pretended it was nothing. Reviving that deep-dyed, centuries-old contempt for the Irish, they have dismissed it with an imperial fly-whisk as a minor irritation. No longer.

On 14 December, the time comes when the EU decides whether the UK has made “sufficient progress” on cash, citizens’ rights … and the Irish border. This roadmap was long ago agreed, and yet as the day approaches there is no plan for that 310-mile stretch with its 300 road crossings. The Irish government, which never wanted the UK to leave, demands, as it always did, that no hard border disrupts trade and breaks the Good Friday agreement.

Why would they expect anything else, when Theresa May herself made that one of her “red lines”? But she made three incompatible pledges: no single market, no customs union and no hard border, an impossible conundrum no nearer resolution than the day she uttered it. Labour’s Keir Starmer keeps pointing to the needless trap she jumped into: why not, like Labour, keep those options on the table?

The Brexiteers turn abusive: the Irish are holding Britain to “ransom” and “blackmail” by conducting an “ambush”. The Sun leader told the taoiseach, Leo Varadkar, to “shut your gob and grow up”, and to stop “disrespecting 17.4 million voters of a country whose billions stopped Ireland going bust as recently as 2010”. Brexit fanatic Labour MP Kate Hoey yesterday adopted a Trump-style demand that Ireland builds a wall and pays for it – for a border they never wanted. The Ukip MEP Gerald Batten tweeted: “UK threatened by Ireland. A tiny country that relies on UK for its existence …” and: “Ireland is like the weakest kid in the playground sucking up to the EU bullies.”

Q&A Why is Dublin opposed to the idea of a hard border? Show Ireland’s taoiseach, Leo Varadkar, has been much more sceptical than the UK about the potential for avoiding border posts via virtual checks on importers. Whilst agreeing with British ministers and EU negotiators that it is inconceivable for there to be a return to a hard border with the north, Dublin argues that the best way for the UK to achieve this would be by permanently remaining in a customs union with the EU and seeking single market membership like Norway through the European Economic Area. The UK has conceded that some of this will be necessary in its interim phase after Brexit, but hopes clever technological solutions can allow it have looser economic links in the long run. Varadkar is not alone in being sceptical about whether such a cake-and-eat-it customs and trade strategy is viable.

Brexiteers, thrashing around, accuse the Irish of using the border crisis as a devious plot to further a united Ireland. But Varadkar rightly says he is not using a veto. There is complete unity among the EU 27: no hard border, loud and clear. He is right not to let this slip to the next stage without a written-in-blood pledge. Brexiteer arrogance chose to think the Irish border was a mere bagatelle – a general attitude forever pretending any obstacle can be resolved in our favour, because we say so. But the EU works on rules and laws, long drawn up together, not to be discarded to gratify the whims of hostile departers. “We won the war – and here’s yet another Churchill movie to prove it,” seems to be their foghorn diplomacy.

The DUP also insists on no hard border, rightly resisting any suggestion of moving customs posts to Irish ports: most of Northern Ireland’s trade moves across the sea to the rest of UK, not to the Republic. Above all, the symbolism of any special status for Northern Ireland that divides it from the mainland cuts to the marrow of its sense of identity, more deeply than all the identity and sovereignty emotions that plunged us into this Brexit morass in the first place.

But the DUP position is, in other ways, a complete nonsense. Why did it back Brexit, without thinking about all this? It still backs Brexit, when 56% of the Northern Irish voted remain. Could it change its mind and choose the least bad option when confronted with an immovable object? It’s time the DUP resolved this by telling the May government – which only survives with its support – that the whole UK must stay in the single market and the customs union: Brexit still happens, but softly. That’s the only solution.

Seeking flexibility among some of the most rigid of UK politicians might seem like tilting at windmills. But we should consider the obligations of Sinn Féin too. Isn’t it time it reviewed its age-old stance? Seven Sinn Féin MPs are elected to Westminster but refuse to take their seats as it involves taking an oath of allegiance. Since 1918, its abstentionism has been unshakable. But right now that stand sees them throw away power and influence while the DUP rides high at Westminster. Think how strong these Sinn Féin remainers would be if they took up their seats and helped to tip the Brexit balance.

As it is, with no assembly and no Westminster presence, they fail their voters. Instead they could help secure an open border, emerging as heroic protectors of Irelands north and south. The oath? Surely they can mouth it and shrug. These are wishful thoughts, but what historic irony if, in the end, the Irish ended up riding to our rescue, saving us from our lemming-like act of suicidal folly.

Any move by them would be savaged by the increasingly anxious Brexit lobby – but it would chime with the gradual change in public opinion. Peter Kellner, YouGov founder, points to its recent polls moving slowly but surely every month since August: the last showed 53% saying the Brexit decision was wrong, 47% right, with the biggest shift in C2DE voters. Some 64% say the government is handling negotiations badly, compared with 21% who think it’s doing well.

Ben Page of Ipsos Mori, another company moving its HQ to Brussels, finds the same slow but steadily consistent movement: those who say staying in the single market matters more than controlling immigration now account for 49% of voters, with a diminishing 37% putting migration first.

These are but straws in the wind: until regret moves to a firm 60:40 for remain, the politics won’t change. There will always be a passionate phalanx of Brexiteers, but the longer transition lasts, the more people will see the price is not worth paying. That’s why the Brexiteers want to dash for any deal or no deal before 29 March 2019 – they know time is not on their side. Sooner or later, the great dam of public opinion will burst. It may take lorries backed up hundreds of miles, or supermarket shelves emptying of some European foods – or just a steady realisation of the real cost.

May delays defining her view on trade as long as she can, as one side or the other of her party will explode when she does. The Irish may force it from her: hard border or customs union? The DUP should use its muscle, while Sinn Féin should ride back into Westminster to a warm remainer welcome. Impossible? Nothing is these days.

• Polly Toynbee is a Guardian columnist