The "Irish question" has come to the forefront of the Brexit negotiations, even though the formal discussion about the shape of Britain’s future relationship with the European Union has not yet even begun.
With Theresa May's Withdrawal deal having suffered a crushing defeat in the Commons, it is still not clear which type of Brexit the UK will pursue – a no deal version in which the UK diverges from the EU model in search of global trading advantage, or the softer version in which the UK remains closely aligned with the EU – but the outcome of that choice will have profound impacts for the UK’s constitutional settlement.
This is true not just for Northern Ireland and its stuttering peace process, but potentially for the way that all of the UK’s other devolved constituent parts relate to each other and the political centre in Westminster.
With the help of academics, businessmen and government sources, here we look at the deep constitutional conundrum posed by Brexit for Northern Ireland, the possible solutions and the implications for the future of Britain.
So what is the problem in Northern Ireland?
The Northern Ireland peace settlement is founded on a central ambiguity: Nationalists can opt to hold Irish citizenship (and only Irish citizenship) while Unionists can remain British. In essence, Nationalists and Unionists can live together in parallel constitutional realities.
Or as the Good Friday, or Belfast Agreement, puts it, it is “the birthright of all the people of Northern Ireland to identify themselves and be accepted as Irish or British, or both, as they may so choose”.
By taking Britain out of Europe, Brexit collapses this core ambiguity: when Britain leaves the European Union in March 2019, the border between Northern Ireland and the Republic of Ireland will become an external border of the EU.
At that moment, the border, currently rendered invisible by the Good Friday Agreement, becomes visible. Both London, Dublin and the European Commission are agreed this would be a disaster for peace in Northern Ireland.
The problem is how to avoid this happening.
What is the UK solution?
London’s idea is to create an "invisible" north-south border based on what the UK government’s position-paper on Northern Ireland calls a “highly streamlined customs arrangement” that would flow from a zero-tariff trade deal between the UK and the EU.
This would be combined with “technology-based solutions” to tackle other issues that arise from Britain’s decision to leave the legal and regulatory umbrella thrown up by the EU’s single market and customs union.
The issue of local traders – say milk producers who raise cattle in the north but process their milk in the South – would be covered by a "local trade exemption". The British argue that such trade is internationally insignificant and follows a long history of a certain amount of flexibility along the 310-mile border, which has some 300 crossing points and has always been a hotbed of local smuggling.
Through the combination of some "imaginative" thinking on the EU’s part, the deployment of technology and the signing of a zero-tariff trade deal, the British have argued they can make good on a the joint pledge not to restore a "hard border" in Northern Ireland.
What is the problem with that?
Put simply, according to both Dublin and Brussels, it just won’t work. When the UK becomes a non-EU country, it will be free cut to trade deals with other countries like the US. That could mean, to take just one topical example, importing hormone-raised beef, genetically-modified food stuffs and chlorinated chicken from the US, all of which are currently blocked from the EU.
A free-trading Britain could also be free to seek competitive advantage by adopting less onerous standards for workers, greater state aid for businesses, laxer environmental regulation and a whole host of other regulatory aspects that impact what the EU calls the "level-playing field".
However far the UK diverges, Ireland will have no choice but to "police" the border; indeed it will be legally obliged to do so by the EU.
Both the Irish government and the European Union dismiss the UK’s technological solutions as "magical thinking". They say that even a "light-touch" scheme will require CCTV cameras and some manpower to carry out spot checks on the 30,000 people and 6,000 lorries that cross the border daily.
Philip Hammond, the Chancellor, recently conceded to the Commons treasury committee that such cameras would become a "legitimate target", but said that they could be set back from the border. He also pointed that fuel, tobacco and alcohol duties are not harmonised north and south, but that had not created border infrastructure.
But the Irish government is adamant that such solutions could never cope with the complexities of a border between Ireland and a fully independent UK, and warns that any infrastructure will revive border tensions and deeply divisive memories.
As one Irish officials puts it: “What happens when one of those cameras gets taken down? Do you send a man to put it back up? Does that man needed to be protected by another man carrying a gun? And if that camera gets taken down again, do you send a man with a gun to protect it? The reality is that any border is a hard border.”
(A recent report by the Legatum Institute, a favourite UK government think-tank, suggested using drones to police the border which EU officials held up to ridicule as proof technology could not solve the Irish border issue.)
So what is the Irish solution?
The Irish solution is to make sure that regulations on both sides of the border between north and south do not diverge. This 'all-island' solution, marked out in a recent European Commission discussion document that was leaked to The Telegraph, would remove the need for a border, since regulations on both sides would remain the same.
As the Irish PM Leo Varadkar put it to the Irish parliament: “We think it best if the UK, either on behalf of all of the UK or on behalf of Northern Ireland, commits to regulatory equivalence, that is to say, that we will operate the same rules and regulations. Without doing that, it is almost impossible to avoid some form of hard border.”
In plain English: either the UK remains in the EU customs union and single market - something repeatedly ruled out by Theresa May because it forsakes the UK’s right to an independent trade policy - or the UK carves out a special status for Northern Ireland with the UK.
What’s the problem with that?
Well, assuming that Mrs May is not going to commit the entire UK to remaining in the EU customs union and single market (which would obviate the point of leaving the EU at all for most Brexiteers) the only option is to give Northern Ireland special status and allow it to remain inside the customs union, or take steps to that effect.
That would, to all intents and purposes, make Northern Ireland a regulatory “exclave” of the European Union, with the government in Belfast essentially required to mirror EU rules, which are in turn handed to Dublin from Brussels.
It would also, by logical extension, require a border between the UK mainland and Northern Ireland in order to preserve the legal integrity of that regulatory convergence - how else could Dublin and Brussels be sure that chlorinated chicken or unfairly subsidised steel, for example, was not illegally entering the EU?
For Unionists, who see Northern Ireland as part of the United Kingdom, the creation of an ‘east-west’ border would be a profound act of betrayal by the Conservative Party.
Arlene Foster, the leader of the Democratic Unionist Party (DUP) has accused Mr Varadkar of being “reckless” by proposing an ‘all-island solution’, while Nigel Dodds, the party's deputy leader, has warned creating a border in the Irish Sea would be “gravely destabilising” to the UK government.
Sounds like deadlock. So what is going to happen?
No-one can predict for sure, but many businesses, political and constitutional analysts and officials closely involved in the technical aspects of any solution spoken to by The Telegraph, argue that the political and commercial logic of the issues thrown up by Brexit point to the creation of an east-west border.
Officially, an east-west border remains a ‘red line’ for the May government, who are supported by the DUP in Westminster, but ultimately - perhaps after an extended period of transition which the Irish government has suggested - some form of unique new settlement for Northern Ireland seems inevitable.
As Colin Harvey, the law professor at Queen’s University Belfast noted recently, whatever way you view Brexit and the Irish question, it unavoidably creates a “major constitutional moment” for Northern Ireland.
Putting aside the fact that 56 per cent of Northern Ireland inhabitants voted to remain, the Good Friday Agreement, with its dual citizenship and arrangements on policing and cross-border co-operation, already create a ‘special status’ for Northern Ireland that enable the peace process to work.
“David Davis talks about ‘constitutional integrity’ but what this misses is that we are no longer living under the constitutional settlement of 1997, we have seen the evolution of devolution,” says Prof Harvey. “Surely one of the lessons of the peace process is that we are already supposed to have especial constitutional process in Northern Ireland?”
Such an outcome might be dismissed as impossible given that the DUP currently holds the key to Mrs May’s parliamentary majority, but the facts on the ground in Northern Ireland are not entirely clear-cut, given the economic cost of a hard border.
Peter Mandelson, a former Northern Ireland secretary, argued this week that Brexit forces a collision of economic and political interests for Unionists that, if not properly managed, risk creating a momentum for the very united Ireland they seek to prevent.
"If you ask people to choose between economic and political priorities, over time you are likely to see a drift towards putting economics before politics and that would encourage support for a united Ireland,” he said at an event in Brussels.
Northern Ireland remaining in the customs union would be the only way, Lord Mandelson ventured, to avoid a hard border and that would be “a huge provocation and boost for the Republican movement” and could ultimately force Unionists to make an “invidious choice” between their political views and economic interests.
Then there are the commercial and practical pressures: put simply, creating an east-west border to manage the UK’s future divergence from the EU is simply much easier than doing it north-south.
For a start, as one senior UK official privately observes, installing number-plate recognition cameras at Holyhead or Fishguard - where “no-one is going to shoot the cameras out in the middle of the night” - is clearly easier than in the Irish borderlands.
Such camera technology on the UK side would be indistinguishable from other toll and speed cameras and soon pass without notice.
Business groups also report that surveys of their members show they would rather see an east-west border, than an north-south one, even though the value of goods sent from Northern Ireland to Great Britain, at £10.7 billion a year, is currently nearly four times the amount sent south.
A paper from the CBI this month in response to the UK government proposals on customs after Brexit appeared to suggest that it too believed the UK solution was undeliverable, begging the question of whether an east-west border might be the only practical alternative?
The paper noted “particular concern” for the future of trading relationships on the island of Ireland and warned that UK solutions “have not been sufficient to ensure the government can deliver on its principle of no hardening of the border”.
For Stephen Nelson, the managing director of Surefreight Global Forwarding, a Belfast haulier, an east-west border is “easily preferable”, even though 70 per cent of his business goes to the UK mainland.
“An east-west border is just far simpler. The 2.5 hours of the ferry crossing enable the driver to get all the paperwork sorted online, and does away with the need for border checkpoints,” he says.
Hauliers’ interests aside, the creation of any kind of north-south border would incentivise Northern-based businesses simply to relocate to the Irish Republic, putting more pressure on local politicians to prioritise jobs over politics.
“We are already looking at setting up an office in the south,” Mr Nelson adds, “because many of our clients have made clear they will move south as a contingency plan in the event of a border arriving, or there being no deal at all.”
For agri-businesses, like LacPatrick, a milk-co-operative that has processing facilities on both sides of a border where a total of 600 million litres of milk crosses in both directions each year, avoiding regulatory disparities and north-south price differentials caused by differing tariff regimes, is even more essential.
“If you have two regimes and a huge disparity in the price of certain things between the UK and the EU, this is going to give a huge opportunity for illegality which undermines legitimate business and only give rise to more illegality,” says Gabriel D’Arcy, the chief executive of LacPatrick who as a young man patrolled the now-defunct border for the Irish Army.
“The absolute priority for the sake of the dairy industry must be in retaining as the greatest possible continuity of existing arrangements, particularly here in Ireland.”
What would that mean for the rest of Britain?
A new settlement for Northern Ireland, in which the North remains in the EU single market and customs union - whether formally or simply by dint of a commitment to retain regulatory convergence - might be ‘practical’ and logical, but it throws up a host of constitutional conundrums.
If the North in effect becomes a regulatory exclave of the European Union, that raises the question of whether Dublin - a foreign power - will effectively find itself representing the interests of part of the United Kingdom at the European Council?
It would also create a new level of special treatment for Northern Ireland that could, the constitutional expert Professor Harvey points out, prompt a much wider discussion about how the other devolved parts of the United Kingdom will relate to Westminster and Europe.
If Northern Ireland is exempted from the consequences of an ‘English’ decision to diverge from the European framework, how long before Scotland asks for its own special dispensation?
The need to address the Northern Ireland question also goes to the heart of the debate currently roiling the Cabinet over how far the UK should really diverge.
If UK consumers are against chlorinated chicken and GMOs - as Michael Gove has suggested - then it may turn out that creating and managing an east-west border is not nearly as difficult and controversial as it currently seems.
But as Wilbur Ross, the US trade secretary, made clear this month, if the UK wants a trade deal with the US, it must be prepared to reduce the “unnecessary divergence in regulation and standards" between the US and the EU that currently keeps cheap US-reared chlorinated chicken out of British supermarkets.
Unfortunately Michel Barnier, the EU’s chief negotiator, made precisely the same point - in the opposition direction. “There will be no ambitious partnership without common ground in fair competition, state aid, tax dumping, food safety, social and environmental standards,” he told a conference in Brussels organised by the UK-based Center for European Reform think-tank.
These are the tough choices that await the British public and their politicians in the New Year.
And the conundrums are not confined to the British end of the discussion. They will equally raise taxing legal and constitutional questions for Dublin and Brussels, which have acknowledged the need to show “imagination” to deal with the questions posed by Ireland.
It is still not obvious the European Commission will legally deliver a settlement in Ireland which - if Mr Varadkar gets his way - will leave a non EU member state effectively remaining as part of the Union’s legal order.
For Dublin, a converged regime between North and South creates problems of its own - not least whether that creates a different kind of border between Ireland and Europe, particularly if there is future divergence between Northern Ireland and the Republic.
None of these questions as yet have answers. But while the challenges for Northern Ireland are clearly massive, concludes Professor Harvey, but they are not insurmountable, as the signing of the Good Friday Agreement itself testifies.
“The Good Friday Agreement demonstrated it was possible to find remarkably imaginative solutions and created 10 years of power-sharing government despite the political obstacles,” he says. “That showed what was possible. We need that same spirit to be rekindled again.”