A certain genius enabled Seán Quinn to make his name in the Irish borderlands. A farmer’s son who left school at 14, he turned a region of dark hedgerows and stony farms, poverty and emigration, into a thriving industrial hub.

He started by selling gravel from the family farm and expanded, creating one company after another. Quinn Cement, Quinn Glass, Quinn Energy, Quinn Manufacturing, Quinn Thermos, Quinn Packaging, Quinn Rooftiles, Quinn Building Products, Quinn Innovation Academy, the name so ubiquitous that some signs simply said Q.

The empire straddled the border between County Fermanagh, in Northern Ireland, and County Cavan, in the Republic of Ireland. Despite disruption during the Troubles, it created thousands of jobs and a £3bn fortune that made Quinn the 177th richest person in the world.

Then he made a disastrous bet on a toxic bank and in 2011 he lost control of his companies. He was declared bankrupt. A vertiginous rise and fall, but not the end of the Quinn story.

Last month masked men abducted Kevin Lunney, a father of six, from his home near Derrylin, in Fermanagh. They slashed his face, broke a leg, tore out fingernails and poured bleach on the wounds. Before dumping him bloodied but alive on a road across the border, there was a final, sadistic touch: they carved QIH on his chest. Quinn Industrial Holdings.

The attack on 17 September shocked Ireland. The cruelty, the barbarity. And the fact it turned out to be part of a wider campaign of intimidation against executives who run remnants of Quinn’s empire.

Lunney, 50, is the operations director of QIH. In 2014 the company acquired Quinn Building Products and Quinn Packaging with money from US-based private equity funds. Many managers, like Lunney, are local men who used to work for Quinn.

Since 2011 they have experienced about 70 incidents of intimidation: vehicles stolen and burned; Facebook groups calling them usurpers; roadside signs branding them traitors; a pig’s head left on a doorstep; an assault in a cafe.

“The states on both sides of the border have been challenged as to whether the rule of law still applies here,” said John McCartin, a non-executive director. “If people can be run out of town by a criminal entity, well, the states have surrendered the region.”

The saga is playing out amid unprecedented scrutiny on the border, with technocrats and politicians from across Europe and the US visiting to study the great riddle of Brexit. Yet few outsiders have noticed that this bucolic landscape – an inspiration for Seamus Heaney – harbours a sinister mystery worthy of Cosa Nostra.

Police said a gang of professional criminals abducted and tortured Lunney. The question is why. Nothing was stolen, no ransom demanded.

“The untouchables in the region have to be called to account for their actions. They’re still untouchable. No one has been arrested,” said Father Oliver O’Reilly, the parish priest of Ballyconnell, a town just inside the republic.

Father Oliver O’Reilly, parish priest of Ballyconnell, County Cavan Photograph: Rory Carroll/The Guardian

O’Reilly caused a sensation on 29 September when from the pulpit of Our Lady Of Lourdes church he denounced Lunney’s assault as a “modern form of crucifixion” akin to those deployed by the likes of Islamic State. “This tactic is used to frighten, intimidate and terrorise ordinary people.”

The priest said a “godfather” was overseeing a mafia-style campaign against QIH executives. He said he and the community should have spoken out long ago. “Friends, there is now an obvious cancer of evil in our midst … we can no longer behave like the ostrich by putting our heads in the sand and pretend that life is normal in our area.”

The congregation gave thunderous applause. When the Guardian accompanied O’Reilly around Ballyconnell last week, parishioners sought him out to shake his hand.

Not all welcomed the homily, he said. “A small section of the community has other loyalties. They told me it wasn’t my place to say such things.”

Quinn, 71, who lives in a gated mansion in Ballyconnell, did not respond to an interview request for this article, but he told a local radio station, Northern Sound, that the attack was “barbaric” and that he feared he and his family would be blamed. “Some people will look in our direction at it,” he said.

The fallen tycoon vehemently denied any responsibility for the assault or the intimidation against his former proteges. “These guys pushed me out, sacked me three and a half years ago, and I’ve had no correspondence, no dealings, no arguments, no fights or anything with them since. It’s a pity that people are coming back to me all the time and asking me to condemn it. Sure of course I’ve condemned it dozens of times.”

At his peak Quinn owned a private jet – registered G-KWIN – and was busy expanding into insurance, banking and hotels across Europe and Asia. There was talk of him bequeathing €1bn to each of his five children.

An audacious gamble on Anglo-Irish Bank proved catastrophic when the bank’s share price collapsed in 2008, wiping out much of the Quinn family fortune and embodying the Celtic Tiger’s economic crash. Quinn was declared bankrupt in 2011 and a year later jailed for three months for contempt of court after trying to block the seizure of international assets. Upon his release and return home, a forklift truck held up a giant poster saying “Welcome home Seán Quinn”.

Facebook groups and roadside signs denounced the executives who took over from Quinn as “grabbers” and demanded Quinn’s reinstatement. Several hundred people attended a rally in support of Quinn in 2018.

Intimidation escalated this year. In February a man in a service station cafe broke Lunney’s nose and threw hot water in the face of Dara O’Reilly, QIH’s chief finance officer. In May an anonymous letter threatened company executives with a “permanent solution”.

Liam McCaffrey, chief executive of QIH, said there had been “gaps” in the police response, but he welcomed the greater number and visibility of officers in the area since Lunney’s ordeal. The company would continue, the CEO vowed. “We try to justify to our families that we’re right to keep going. We don’t want to become hermits.”

A sign protesting Seán Quinn’s exclusion from Quinn Industrial Holdings. Photograph: Rory Carroll/The Guardian

A few hundred yards from the company’s HQ a roadside sign accuses McCaffrey, Lunney and his brother Tony, another director, of having inflated salaries (McCaffrey calls the figures exaggerated). Underneath, in red, the sign adds: “Seán Quinn zero £pounds.”

The sign has stood in front of a tile shop for several years. When the Guardian asked the shop’s employees about it, they looked blank. “We know nothing about it.”