In March 2017, the EU's chief Brexit negotiator Michel Barnier was standing in a field in South Armagh, just above the border between Ireland and the U.K. province of Northern Ireland. Peter Sheridan, his host, pointed into the distance and asked: "You see the border?" To which Barnier replied, "No, where?"
"That's the idea," Sheridan said.
The "Irish question" has emerged as an intractable thorn in Brexit negotiations. U.K. Prime Minister Theresa May faces a momentous day in Brussels on Monday — the EU has given her government until December 4 to "put a final offer on the table" for a solution on the future of the Northern Irish border before any trade talks can proceed.
Uncertainty over a potential hard border between Ireland and its northern neighbor has evoked memories of a darker time before the signing of the 1998 Good Friday Agreement, which ended large-scale sectarian violence in the border region and allowed for the free movement of people and goods.
The U.K. wants to exit the EU customs union, taking Northern Ireland along with it, something many fear cannot be done without the imposition of a physical border. A cornerstone of the EU, the customs union exempts EU member states and select outside territories from paying customs duties on all goods traveling within the union.
Concerns about renewed sectarian tensions and political divisions surrounding a possible border are now front and center.
"The border between Ireland and Northern Ireland is no longer a symbol of division, it is a symbol of cooperation. And we cannot allow Brexit to destroy this achievement of the Good Friday Agreement," European Council President Donald Tusk told press on Friday.
Ireland's government has staunchly opposed the prospect of a hard border and asked that Northern Ireland remain in the customs union, threatening to veto progression of Brexit talks unless Westminster offers a sufficient solution to the current impasse.
Peter Sheridan spent 33 years as a police officer in the Police Service Northern Ireland — formerly the Royal Ulster Constabulary — and oversaw Northern Ireland's murder unit until his retirement in 2008. He now heads Co-operation Ireland, a U.K., Irish and U.S.-registered charity working to promote dialogue and practical collaboration between Irish and Northern Irish communities.
"I moved out of my house twice when people who didn't like me thought that I should go to the next world faster than I wanted to. My house had fortified armored glass and other security measures … And this went into the 90s," Sheridan told CNBC in a telephone interview.
Nearly two decades after the signing of the Good Friday Agreement, which was underpinned by the EU and effectively ended more than 30 years of conflict known as "The Troubles", the progress is obvious. Large scale violence has ceased, as well as the threats to Sheridan's life.
But pockets of trouble persist.
"There are still people who see physical force as a way forward — they're small in number but can be dangerous. You're still seeing young people shot in the legs, people who aren't conforming on both sides. The violence is certainly reduced, but it still exists," he said. "Sectarianism is alive and well."
The lengthy Northern Irish guerrilla war over the province's constitutional status saw more than 3,500 people lose their lives in bombings, indiscriminate shootings and assassinations carried out by both Irish nationalist and Northern Irish loyalist paramilitaries like the Irish Republican Army and the Ulster Volunteer Force, among others. British troops were deployed to the conflict as well.
Northern Ireland's majority Protestant population, which largely identified as British, had no desire to live as a religious minority under an Irish government. Meanwhile, Catholics living in the northern province, who saw themselves as Irish, suffered persecution at the hands of Northern Irish authorities and sought a united Ireland free from British rule.
"What you got with the peace agreement was an agreement on the systems and institutions of government," Sheridan said, describing the Good Friday Agreement's fundamental achievement of a power-sharing system for the region's political and religious groups.
"The question is how do you underpin that politically, by normalizing relations in society where people who were former enemies learn to live together as citizens? That's what we needed to be doing next."
Some 95 percent of social housing in Northern Ireland is segregated along religious lines, Sheridan explained. And only 6 to 7 percent of children and adolescents attend integrated schools.
The New York Times reported that a new integrated housing development built in a Protestant area has only led to Catholics being intimidated out of their homes — at least 30 people are left homeless annually for this very reason, the NYT discovered.
"Some paramilitary organizations still have coercive control and influence over some communities," Sheridan pointed out. "You have to ask, what are they doing around 20 years after the ceasefires?"
While the vast majority of Irish Republicans were Catholic, and most Unionists, who saw themselves as part of Britain, were Protestant, the conflict was not fundamentally a religious one.
"It was a conflict of identity," Sheridan explained. "Some people saw themselves as Irish, and their allegiance was to Dublin; some people saw themselves as British and their allegiance was to London.
"What the Good Friday Agreement did was to give people the identity they wanted." Sheridan has made a point of holding both Irish and British passports to embody this. "Brexit poses the danger of unraveling that."
"If people feel that their sense of identity is being diluted by creating a border between them and the country they feel they identify with, you're inevitably going to create tensions."
Conor Patterson, head of local business organization Newry and Mourne Enterprise Agency, has dedicated his career to improving employment prospects in his district, which sits just above the Irish border.
"Having experienced that hardest of hard borders, we don't want to see anything return that will put barriers to movement," Patterson told CNBC.
He recalled when his town of Newry, once a flashpoint of sectarian violence just on the north side of the border, was at 30 percent unemployment in 1972 — the highest in Western Europe. Now it's at 2 percent.
"The transformation has been remarkable. I attribute the transformation to a number of factors — important among those was the dissolution of the border as a barrier to the movement of goods and people."
Growing up, Patterson would frequently drive to Dundalk, south of the border, to visit his mother's family. "The road we had to travel was formerly the most dangerous road in Western Europe, there were so many attacks," he recalled. "Now, it's an open motorway. It was completed in 2008, and the border link was 40 percent from the EU."
Newry's last military watchtower was removed in 2003 — the town still witnessed bomb scares until 2010.
"Once there was no barrier to movement, that allowed people to relax a bit with regard to where they sat in terms of their nationality," Patterson remarked.
"My ambition is stability. My ambition is to get back to what my job is about, and that's creating employment. We just want to get back to living the life we had before this crisis," he said.
"Ireland and the U.K. were each other's best allies within the EU," Patterson added. "For Ireland to be so at odds with the U.K. should be concerning to the U.K."
At a newly opened cafe in central Belfast, Sinn Fein lawmaker and former Belfast mayor Mairtin O Muilleoir recounted his first days in politics in the 1980s.
"Thirty years ago this month, I started my first day at Belfast City Council. I wore a flak jacket — you had to," he said. "Our offices got blown up, I had many friends shot, some killed." Sinn Fein was the political wing of the Irish Republican Army, and is currently the second-largest party in the Northern Ireland Assembly by one seat.
With the new hotels and stunning Titanic Museum in the Northern Irish capital's central Titanic Quarter, it's hard to imagine a time when the location was a war zone.
"Where we were was a nightmare," O Muilleoir recalled. "And the progress you see all around us, the resurgent city center — all that's built on the foundation of peace, of reconciliation, of trying to find a compromise. That compromise was the Good Friday Agreement, which placed us clearly within the EU."
"I think that any attempt at replacing borders on the island of Ireland is a mistake, not just because it's a threat to the peace process, but because it damages the process of reconciliation, of the healing of wounds, of building a new type of society," O Muilleoir stressed.
Gary Bassett grew up in central Belfast, and was a teenager when the Good Friday Agreement was signed.
"I think they're just exaggerating a bit about it, it's a bit overdramatized," he told CNBC while driving his taxi in the Northern Irish capital. "None of the young people want this. I think the older generation keeps it up."
"I'm 37, but I can remember what it was like. My parents wouldn't take us into town for a meal or a drink because you never knew when the next bomb would go off."
Bassett maintained that his own generation wants peace. "I have kids," he said. "They don't even know the difference between Protestant and Catholic."
"Young people are our saving grace, and young people overwhelmingly wanted to stay in the EU, because they're outward looking," O Muilleoir said. He lamented Brexit as what he viewed a betrayal of the U.K.'s youth, as the vast majority of those under the age of 25 did vote to remain in the EU. He also noted that 56 percent of Northern Irish voted to remain as well.
Ulster Unionist Party (UUP) leader Robin Swann rejected the possibility of revived sectarian conflict. "That's what we hoped the last 20 years would be about — moving away from that civil conflict. I think we're far enough, we're mature enough in Northern Ireland to realize that this has to be solved politically," he told CNBC in Belfast.
The UUP is the fourth-largest party in the Northern Ireland Assembly with 10 seats. Unionist parties support leaving the customs union along with the rest of the U.K.
David McCarthy, a bartender at downtown Dublin's high-end Marion Hotel, agreed: "If you ask me, it's over and done with. I'm 25, and I talk to 35-year-olds who are still bitter. But for my generation, it's in the past. We joke about it."
Patterson, however, wasn't entirely convinced by the optimism of some of his more youthful counterparts.
"Is this exaggerated? Maybe younger people don't see all the angles," Patterson warned. He pointed to Sinn Fein MP Chris Hazzard, who in late November warned that civil disobedience could arise in response to a hard border.
"I'm old enough to remember how civil disobedience escalates," he said. "The point about escalation is that it doesn't have to be pre-planned."
Politicians, meanwhile, maintained their desire to put an end to any ongoing violence.
"In terms of those who are still involved in violence, extortion or criminality, it's our intention to face them down," O Muilleoir emphasized.
"Those of us who have experience the peace over the last 20 years recognize it as the greatest gift ever given to this country, a gift which absolutely transformed our community prospects, our cultural prospects, our economic prospects. We refuse to contemplate any collapse, any return to the bad old days."
Sheridan estimated more than a thousand people are alive today because of the Good Friday Agreement. And despite having been at the front lines of sectarian conflict for the majority of his life, he says he is optimistic about a future deal.
"I'm 95 percent sure there won't be a hard border," Sheridan contended. "It is in nobody's interest — the British government, the Irish government, unionism, nationalism — to have a hard border."
Europe awaits the plans proposed by Westminster, although Ireland, with the EU firmly behind it, will be the judge of that plan. Tusk may be correct in his Friday assessment that "the key to the U.K.'s future lies — in some ways — in Dublin, at least as long as Brexit negotiations continue."