WICKLOW, Ireland — When is a border not a border?

That is the politically-charged riddle Brexit negotiators will grapple with when they come to discussing Northern Ireland and its frontier with the Republic of Ireland to the south.

Prime Minister Theresa May's vision of a hard Brexit — in which Britain leaves the single market and the customs union — makes it hard to see how reinstating some kind of border will be avoided since Northern Ireland would become the only land border between the EU and the U.K.

However, as former U.K. Prime Minister Tony Blair pointed out at a conference of conservative European People's Party (EPP) politicians near Dublin Friday, the stakes are extremely high.

A hard border would be a "disaster", he told MEPs, and would endanger the delicate peace brought in by the 1998 Good Friday Agreement his government negotiated.

Blair told reporters that a creative solution will require “good will, and a lot of ingenuity and innovation, and maybe the use of technology” to avoid a return to the old frontier that was so divisive in the past.

Back in Brussels, there is broad political will to find a solution, despite few concrete suggestions as to how this would work. "You’ve got no idea how creative the EU can be when there’s a widely shared political goal,” an EU diplomat said.

Preserving peace

During the Troubles, the hills around the Republic of Ireland's border with Northern Ireland were known as "Bandit Country," a militarized zone that marked a key battle-ground in the IRA's conflict with the British state. Border checkpoints became targets for paramilitary fighters and organized crime and smuggling was commonplace.

Over the course of three decades, the Troubles claimed the lives of more than 3,600 people with thousands more injured.

There are concerns in Brussels that any sign of a physical border in the form of posts or checkpoints could again become a target for attacks by dissidents and organized criminals. A hard border would also curb trade and could damage the economy on both sides of the border.

Several delegates at the EPP gathering Friday view a border as inevitable. “I don’t see how a border could be avoided, even if only to ensure security of people and goods,” said Franck Proust, the head of the French delegation of the EPP. “There will certainly be an economic border.”

Ireland's Minister for Food, Forestry and Horticulture Andrew Doyle said "some form of custom check will have to be embraced" after Brexit.

Last month, the Council of the EU pledged in its official guidelines on Brexit negotiations for “flexible and imaginative solutions” with the aim of “avoiding a hard border, while respecting the integrity of the Union legal order.”

One idea that has been floated would be to position customs controls several kilometers back from the physical border itself in order not to draw attention to them.

One senior Commission official also pointed out that larger companies will be able to reduce the impact of customs by participating in the EU's so-called Registered Exporter system, which came into force this year. The system allows countries outside the EU to reduce customs fees by certifying themselves online and submitting their own statements of origin.

A more ambitious idea is to create a "virtual border" whereby all goods passing from one side to the other would have to be registered online beforehand, with any import tariffs paid at that point. There would be no need to check "paperwork" at the frontier itself because that would all have happened before goods passed through. National auditors carrying out factory inspections would ensure compliance with the system.

It might work in theory but as yet there is no agreement about how such a system might work in practice.

Cherry-picking

Whatever the technical challenges of creating a non-border frontier, the potential consequences are so dire that there is huge political will for a solution to be found.

“Politically, commitments have been made at the highest levels at Europe, U.K. Europe and Ireland that there will be no return to a hard border for very good reasons on this island," Irish MEP Mairead McGuiness said. “Therefore whatever the technical issues or the trade issues, that’s a commitment that must be honored, and for the moment people are really struggling the find a solution,” she said.

However, not everybody agrees that a border must be avoided at all costs. Manfred Weber, head of the EPP group in the European Parliament, said the border question must be linked to the nature of a future free-trade agreement with the U.K. and that the EU27 were waiting to see what Britain's position would be.

“We have to wait for a clear message from the Brits,” Weber said.

“If we agree theoretically on a free trade agreement … and no tariffs, then you can finally find a way to have discussion on borders,” he said. “But we have no idea what they want to achieve.”

Weber emphasized that the EU wouldn’t accept what he described as the U.K.’s “cherry-picking” approach on many issues, including on borders. “They still think that … a kind of an opt-out opt-in mechanism is the future of their relations [with the EU.]"

With trade due to be discussed towards the end of the talks, the Irish border question might be hanging for many months, or even years, to come.