We always knew Ireland would be the meat in the EU-UK Brexit sandwich. Now, here's the incontrovertible proof in practice.
Britain would like any settlement on the border in Ireland to be a template for its border and trading arrangements elsewhere - like the Dover-Calais axis and other links with mainland Europe.
So, EU chief negotiator Michel Barnier's response to that emerging scenario in Brussels yesterday was characteristically blunt.
"The UK want to use Ireland as a kind of test case for the future of the EU-UK customs relations," he said.
"That cannot happen.
"Creativity and flexibility cannot be at the expense of the integrity of the single market and the customs union."
The former French agriculture and foreign minister, and former EU regional commissioner, has great empathy with Ireland's unique predicament.
The Republic has achieved recognition for its interests by getting the border, and the Good Friday Agreement, listed among the first wave of key items to be dealt with in these EU-UK divorce talks.
There was a positive development last week, in the third phase of EU-UK talks.
This was Britain's recognition that free movement can continue between these two islands, and no impediment from London will be placed in the way of Ireland's obligation to guarantee EU citizens' free movement in and out of the Republic of Ireland.
Michel Barnier alluded to it again yesterday.
It is, in sum, a good thing.
The point in practice is that EU citizens moving between the British and Irish jurisdictions - including across the border north and south - could not bring trade products with them.
That is the rub and the ongoing row.
Things are headed towards an outer deadline of March 29, 2019, for the UK's formal exit from the EU.
The nearer deadline is October 19 and 20 next, when EU leaders gather in Brussels for a special Brexit summit.
According to the rather vague negotiating timetable, three key issues: EU citizens' rights, the Irish border, and the UK's financial bill on leaving the EU bloc, would have to be resolved by this upcoming summit.
Only then could talks move on to the issue closer to British interests, and also of vital interest to the Republic.
This is the EU-UK's future trading links, which raise the nightmare spectre of tariffs and customs procedures applying to imports and exports with our oldest and major trading partners.
There is a great deal of chicken and egg surrounding this talks division. Britain keeps saying it really cannot do meaningful talks until it knows the shape of the EU-UK future trading relationship.
But viewed from Brussels, and taking on board the interests of the other 27 EU member states, the issue of Britain's exit bill is paramount.
EU negotiators are implacable on this one: Britain must agree the calculation key for the divorce bill before much else can happen.
The actual amounts being cited - from €40bn to €100bn - only seem huge.
You have to realise that they are based on several years of payments, mostly on budget plans already agreed, and to be paid by the world's fifth largest economy.
There were speculative British media reports through August, and again this past weekend, that Prime Minister Theresa May is ready to settle on something between €40bn-plus and some €55bn.
There is an understandable proviso that no such thing can be done until after the Conservative Party conference in Manchester on October 1.
But while in reality the likely amounts are not huge, the political implications are totally explosive.
Even a settlement sum at the lowest of those ranges has the potential to stoke up rage among anti-EU elements in England, rather than Britain writ-large.
There were stark reminders of this in the British parliament yesterday as the major task of bringing in Brexit legislation began.
This is aimed at incorporating current EU laws into UK domestic law to avoid a huge legal vacuum.
But already anti-EU Conservatives are collecting signatures for a warning to Mrs May there can be no "backsliding" on her stated aim of leaving the EU single market and customs union.
Both sandwich sides are very hard bread.