Just before European Union leaders agreed their guidelines for the Brexit negotiations last week, the president of the EU council, Donald Tusk, said: “It is clear that progress on people, money and Ireland must come first.” It was rather startling to find Irish concerns up there on the list of fundamental priorities, with the rights of EU citizens in Britain and with the settling up of the final bill. And when the guidelines were agreed, it was clear that this was more than rhetoric. EU governments have essentially committed themselves to allowing Northern Ireland to rejoin the EU if Ireland is united.

This is a very big deal. It suggests at one level that Brexit really does mean Brexit – in the very literal sense that the entity that is exiting is Great Britain and not the United Kingdom. There has been a habit of using Britain and the UK as synonymous terms, but of course they are not. The very name of the state – the United Kingdom of Great Britain and Northern Ireland – acknowledges a distinction.

Now, suddenly, the difference is stark. The EU has just done what the Brexiteers steadfastly refused to do – acknowledge that Northern Ireland is not just another British region. The 27 remaining member states have signed up to treat Northern Ireland as a place of its own.

Britain’s departure from the EU is (in principle) to be final; Northern Ireland’s is now contingent. Britain is getting a divorce; Northern Ireland is being offered a trial separation. For Britain, there is a one-way ticket; for Northern Ireland, there is an automatic right of return. The implicit offer is two unions for the price of one: unite Ireland and you reunite with Europe.

We should not downplay the significance of this. Even with the glaring exception of German unity, the EU is not inclined to mess with existing national borders. Spain, in particular, can’t be comfortable with conceding that regions within existing states might be able to forge their own future relationships with the EU.

But in a diplomatic coup for the Irish government, these hesitations have been overcome.

The EU has just done something it has never done before: it has offered an incentive to part of an existing European state to join another state. The language may be quiet but the message is pretty loud. People in Northern Ireland have just been told that if Brexit is a disaster for them (which it may well be), they can vote to rejoin. They will be let back in without conditions or negotiations. Nobody else in the UK has this offer of satisfaction with Brexit or your EU passport back. Perhaps for the first time in its troubled history, being from Northern Ireland is a distinct advantage.

We have to be careful about this, however. What’s just happened is that Brexit has pushed the question of a united Ireland further and faster than the vast majority of Irish people, nationalist as well as unionist, really want to go. Ireland and Europe have been forced into a kind of time travel – we have to delve into a future for which no one is prepared.

Everyone – even Sinn Féin – knows that it is foolish to talk about a united Ireland without talking about a united Northern Ireland first. Forcing a million unionists into a new Irish state without their consent is in nobody’s interest. And it can’t be assumed that even a bad Brexit will suddenly make unionists change their minds about wanting to stay in the UK: political and religious identity often trump economic self-interest.

As for nationalist Ireland, the thing to understand is that there is a vast difference between wanting something in principle and wanting it now. The rubric that should be attached to the aspiration to a united Ireland is St Augustine’s prayer: “Lord, make me pure – but not yet.” Polls have consistently shown support for a united Ireland – but not yet. When asked in November 2015, if they would like to see a united island in their lifetime, 66% of respondents in the republic answered yes. But, asked if they would like the island to be unified in the short to medium term, that figure was cut in half. And in the same RTÉ/BBC poll, just 27% of those surveyed from a Catholic background in Northern Ireland said they wanted a united Ireland in the short-to-medium term.

Striking as it is, therefore, the EU’s implicit support for the possibility of Irish unity should not suck attention away from some much more important words in its negotiating guidelines: “in view of the unique circumstances on the island of Ireland, flexible and imaginative solutions will be required”.

Giving Northern Ireland the right to opt back into the EU through a united Ireland is a good thing. But long before we get to that possibility, there are the negotiations themselves. They can’t be conducted on the vague assumption that a united Ireland will solve the horrendous political, economic and social dilemmas that Brexit creates for the island of Ireland.

“Flexible and imaginative solutions” are exactly what Ireland needs – and relying on a 19th-century concept of a unitary state is neither flexible nor imaginative enough. The EU has taken a huge conceptual leap in explicitly recognising Northern Ireland as a different kind of place. It needs to follow through on that by thinking about what actually makes Northern Ireland different. It is not just its history of violence. It is that stopping the violence meant creating an ambiguous space that is neither quite one thing nor the other: neither simply Irish nor simply British.

What the EU and Britain both need to recognise when the negotiations begin is that this ambiguity is not just a possibility. It is a necessity. Europe is already halfway towards a recognition of Northern Ireland as a place that might still be attached to the EU after Brexit.

It needs to go all the way, and commit itself to the fruitful ambiguity of a Northern Ireland that is both still in the UK and still in the EU. After all, if people in Northern Ireland are being told they can vote to rejoin the EU, why ignore their vote not to leave it in the first place?