CLONES, Ireland — Businesses along the boundary between Ireland and Northern Ireland are gearing up for hardship as Brexit turns it back into an external EU border.

But one sector is looking forward to it: smuggling.

Locals along the complex division, which twists for 500 kilometers through towns, villages and remote countryside, joke that at least Brexit could be good for employment.

“You can have a house with a front door and a back door, and the border goes through the middle. Of course, people will find a bit of ingenuity to get around it,” said Pat Treanor, a Sinn Féin councilor in the border county of Monaghan. “The black market will thrive.”

The bigger the differences between the tax and regulatory regimes on each side of the post-Brexit border, the greater the opportunities for illicit profit.

British Prime Minister Theresa May has said the U.K. will leave the EU's customs union. Complying with the procedures required to cross the border after that happens would add costs to businesses of up to 24 percent, according to the U.K. Treasury — offering an attractive profit margin to anyone willing to break the rules.

Since 1922, when Ireland broke away from the United Kingdom, people have been finding creative ways to get around the border. There's a certain cultural acceptance of exploiting the border, which carries a sheen of political justification and Robin Hood-style glamor.

“It might be seen as populist to smuggle in the community I live in,” said Declan Breathnach, a member of parliament for Fianna Fáil from the border county of Louth.

Cross-border arbitrage runs from legitimate bargain hunting to black-market trade. Older residents of border towns fondly reminisce about smuggling butter or tea to get around food restrictions in place before the 1970s. The later exploitation of EU farming payments by moving animals over the border is even celebrated in song. And although they are illegal in the Republic of Ireland, fireworks smuggled down from Northern Ireland light up the skies of Dublin every Halloween.

Local officials believe the subsistence smuggling of the past has given way to something more serious that could be turbo-charged by Brexit.

“It has moved into racketeering,” said Breathnach, who has introduced a bill in parliament to bring in fines for buying illicit tobacco, alcohol and fuel. “They’re in every racket you can think of, and it’s going to become more acute. I keep raising this issue at senior level and nobody’s listening.”

In all, illicit trade in Ireland may cost the exchequer over €800 million a year and industry €1.59 billion a year, according to the study.

The border has always been porous, even when the resources of the British Army were focused on securing it, because of its complexity and local resistance, according to the armed forces' own accounts.

The smuggling infrastructure and know-how to make the most of Brexit is already in place. Border commerce long ago adjusted to regulatory changes and fluctuations between sterling and the euro.

Roadside shops advertise cheap fuel and alcohol deals, and markets are held in fields straddling the boundary where visitors can buy anything from agricultural equipment to clothing to washing powder from the backs of vans. Some black- or gray-market dealers even offer home delivery.

What gets smuggled?

In recent years, a major trade across the border has been in fuel, with smugglers adjusting their prices according to regulatory changes. A sophisticated industry grew up based around removing the dye from diesel meant for agricultural and industrial use, which is taxed at a lower rate, so it could be sold as more expensive diesel for ordinary cars.

A crackdown on the practice and changes to fuel that make it harder to remove the dye helped reduce the trade, but there are signs the racketeers have now moved into oil for heating houses, according to local officials.

Businesses that sell coal and peat fuel complain that a rise in tax in Ireland caused their business to shift to the black market. The change created a €2,000 incentive for anyone who can move 20 metric tons of coal from Northern Ireland into the Republic, according to the Solid Fuel Trade Group.

According to a study by legal and accountancy firm Grant Thornton, there are between 10 and 12 main gangs with deep local roots who are involved in the cross-border fuel trade. They take advantage of the two jurisdictions in law enforcement to evade capture, moving operations from one side of the border to the other when needed.

In all, illicit trade in Ireland may cost the exchequer over €800 million a year and industry €1.59 billion a year, according to the study.

According to Breathnach, the Louth lawmaker, border gangs are also targeting groceries such as soft drinks and washing powder, mixing and repackaging them before passing them on for sale.

There are clear risks for food safety. The border is an agricultural area, and some farms straddle the international boundary. If the U.K. adopts different standards following its exit from the EU, the border would be an obvious weak point to introduce food illicitly into Ireland, which has invested heavily in quality and traceability systems in recent years.

Farmers are looking for ways to offset potential hardship. Those on the Northern Irish side of the border may lose their Common Agricultural Policy payments and supply chains on both sides will be disrupted.

Paramilitary overlap

During the years of conflict known as the Troubles, the border areas saw some of the worst violence. As peace took hold, police believe that remnants of paramilitary groups on both sides that once fought over whether Northern Ireland should be part of the United Kingdom moved into organized crime.

Locals say there is an overlap between republican paramilitary circles and those in charge of cross-border smuggling. Locals refer to the main figures as “godfathers” and can be reluctant to speak about the subject. (This reporter was warned some smugglers wouldn’t hesitate to “take you out.”)

The name most commonly mentioned is Thomas "Slab" Murphy, a prominent republican whose family farm straddles the border and who was convicted of tax evasion last year and jailed for 18 months. He is widely reported to have been a senior member of the Provisional IRA and to have amassed a fortune smuggling animals and fuel through his farm.

“It would be like telling Germany to rebuild the Berlin Wall” — Pat Treanor, a Sinn Féin councilor

Dublin, London and Brussels all say they do not want a return to the borders of the past. But it is unclear how customs can be avoided. If Northern Ireland is outside the single market and Ireland is within it, they are inevitable.

Peter Leary, a University of Oxford historian who wrote a book about how locals have negotiated the border, noted that customs were never the plan in the past either. Rather, customs were forced into place by the pursuit of other policies.

“The border didn’t come about due to deliberate design,” Leary said.

“What the history points to is that … people pursue other things that make customs inevitable. That leads to other potential dangers, the potential for conflict,” Leary said. “Customs were attacked in the 1920s, they were burnt down in the 1930s, they were attacked in the 1950s … If there is to be a return to customs will they have to be defended? Will there be a re-militarization of the border?”

The British Army attempted to enforce a hard border in the 1970s to combat the IRA. Roads were blown up or blocked to reduce the number of crossings that required manned checkpoints. The attempt was abandoned after the army found the complexity of the border, the manpower required, and the strength of local opposition made the task unfeasible.

Treanor, the councilor, was involved in a campaign to reopen border crossings that the British Army had blocked or blown up. He is missing a finger from one 1994 incident when he was arrested and IRA gunmen fired on the police car he was in. Treanor said people would do “whatever they have to” to get around the border.

“It would be like telling Germany to rebuild the Berlin Wall,” he said. Asked about the risk of a return to violence, he said “it depends.”

“If a customs or whatever border were to be introduced and it turns out there are military, armed people on it, and they’re treating people like they used to treat people," Treanor said, "that would create conditions [for violence].”