LONDON — The question of what happens to the border between Northern Ireland and the Republic of Ireland after Brexit is so difficult to solve that it could ultimately derail Brexit.

Even more than the rights of EU citizens and the size of the UK's Brexit divorce bill, the Northern Irish border is so contentious, and so politically dangerous to tackle, that there may, in the end, be no viable solution.

If Theresa May's Brexit vision is destined to fail, then this could be the one issue that triggers it.

So why does the Irish border matter so much?

The border between Northern Ireland and the Republic of Ireland is currently more or less invisible. There are no border controls meaning goods and people move freely to and from the neighbouring countries.

However, Brexit creates complications. When the UK officially leaves the EU in March 2019, Northern Ireland will be removed from the 28-nation bloc alongside England, Wales and Scotland. The Republic of Ireland, on the other hand, will remain an EU member state.

Why does this matter? Well, if May sticks to her current plans to leave the customs union, then there will need to be some form of new border controls between Northern Ireland and the Republic of Ireland, in order to avoid smuggling between the UK and EU.

The European Union's chief Brexit negotiator Michel Barnier has previously warned that "frictionless trade" is "not possible" following Brexit.

This is deeply important to the UK economy. A House of Lords report published in December said that €60 billion is traded between the UK and Ireland each year, and an estimated 30,000 people cross the Irish border every day. A hard border would put this at risk.

However, the political impact on the Northern Ireland peace process could be much greater.

Surely this won't actually threaten peace in Northern Ireland?

It might. The Good Friday Agreement of 1998 relied heavily on membership of the European Union, with free movement between Northern Ireland and the Republic of Ireland a key part of that.

Brexit will upset this delicate balance of power in the region and risk, as John Major has warned, the return of the "hard men" to Northern Ireland.

"People regard the peace process, that was very hard-earned [as certain]," he warned earlier this year.

"People shouldn't regard it as a given. It is uncertain, it is under stress, it is fragile."

As Irish columnist Fintan O'Toole wrote, Brexit means that "English nationalists have planted a bomb under the settlement that brought peace to Northern Ireland."

The EU's chief Brexit negotiator Michel Barnier at the Irish border Reuters

So what are the options for the border after Brexit?

1. Technology

Theresa May's government has pushed for a technological solution to the border, like the one between Norway and Sweden.

This would mean that vehicles would not have to stop at the border, and it could be monitored remotely.

However, as the Irish foreign minister, Simon Coveney told a meeting of EU foreign affairs ministers last month: "What we do not want to pretend is that we can solve the problems of the border on the island of Ireland through technical solutions like cameras and pre-registration and so on. That is not going to work.

"Any barrier or border on the island of Ireland in my view risks undermining a very hard-won peace process and all of the parties in Northern Ireland, whether they are unionist or nationalist, recognise that we want to keep the free movement of people and goods and services and livelihoods."

There are alternatives, however.

2. Using the sea as a border like it's the Middle Ages all over again

One solution reportedly under consideration by The Irish government is for the Irish sea to become the border between Ireland and the UK, allowing the island of Ireland to become one customs zone.

However, the UK government is not keen. Brexit secretary David Davis told MPs last year that he "did not see [a sea border] would be the solution."

Sir Jeffrey Donaldson, a senior DUP MP also said that his party "would be strongly opposed to the idea that you would create a border in the Irish sea between the island of Ireland and Great Britain."

This is important as 10 DUPs are currently partners with May's Conservative government in a confidence and supply deal in Westminster. Any threat to this informal coalition could bring down the government.

A sea border would also push both parts of Ireland together in what would be seen as a victory by those who want a united Ireland, but a failure for unionists, who would see it as Northern Ireland moving away from the UK.

Irish Taoiseach Leo Varadkar with Prime Minister Theresa May Reuters

3.Continuity

UK and EU officials have both said that they want to continue the Common Travel Area, which has meant that British and Irish citizens can travel between the two countries unrestricted since 1922.

How this would be managed is unclear, as any kind of open border between Northern Ireland and the Republic of Ireland would mean that the UK would still have an open border with the EU, making stopping free movement impossible.

However, it would be possible if May's government decides to change course on the customs union.

Daniel Mulhall, the outgoing Irish ambassador to the UK said that he hoped that the UK would stay in the customs union.

He told BBC Radio 4’s Today programme on Friday: "Ideally, we would like to see the UK remain in the customs union. That would solve many of the problems that arise."

"Ideally we would wish Britain to remain in the EU, that’s not going to happen, we’d like Britain to remain in the single market, that may not happen," he said.

"But we think putting forward our view that remaining in the customs union would resolve many of the issues on the isle of Ireland that seems to us a practical solution."

On Friday, the Irish Taoiseach suggested that a new UK-EU customs union could be the way forward.

There is, of course, one other solution.

4. A hard border

If the UK and the EU do not find a solution to the Irish border, there may have to be a return to a hard border between Northern Ireland and the Republic of Ireland for the first time in twenty years.

It appears that neither the Irish nor the British governments want this to happen, with Davis telling a Lords select committee that "no one wants to go back to the hard border."

A hard border would interrupt trade between the UK and Ireland, but also communities and people's everyday lives, as goods and people are checked as they cross the border.

O'Toole told the Guardian: "The notion that you would even consider implementing a hard border here again seems ludicrous. It is not just that it was so porous even when it was heavily policed, but that it would be read here as the British government not giving a damn about the legacy of the Troubles and the terrific progress of the years since the Good Friday agreement."

Irish socialist Eamonn McCann said that he estimated a hard border would require 10,000 guards, on which he remarked: "Never mind the cost; that would mean there would be 10,000 sitting ducks along the border."

So nobody wants a hard border, but we may have to have one, and even if we don't, we will need to create some new form of not-hard, but not-soft border that hasn't actually been invented yet?

Basically, yes.

This doesn't sound good.

It really doesn't. In fact, unless the UK and Ireland can resolve this question, then Brexit negotiations could come to a grinding halt before they've even got going.