Four members of the same family have been arrested over the abduction and assault of Co Fermanagh businessman Kevin Lunney.
In a joint policing operation the PSNI arrested two men aged 23 and 61 and a woman aged 61 in Co Fermanagh on suspicion of conspiracy to kidnap, conspiracy to cause grievous bodily harm and assisting offenders.
All three suspects are being questioned by detectives in Omagh.
Another man in his 30s was arrested by the Garda in relation to the abduction and assault and is being questioned at a police station in Co Cavan.
In December four men were remanded in custody in the Republic after being charged with the kidnap and torture.
Mr Lunney, a director at Quinn Industrial Holdings (QIH), was on his way home in Derrylin on September 17 last year when he was abducted and taken across the border.
The 50-year-old father-of-six had his leg broken, his fingernails sliced, was doused in bleach and had 'QIH' cut into his chest, before being dumped on a roadside in Co Cavan.
The assault left him with life-changing injuries and was the most high-profile incident in a six-year campaign of intimidation against those who took over the Quinn business portfolio from Sean Quinn.
Mr Quinn, who was once Ireland's richest man, has always condemned the intimidation.
Detective Chief Inspector Julie Mullan from the PSNI's Criminal Investigation Branch said: "The arrests today show the close working partnership between the PSNI and An Garda Siochana and our determination to bring those responsible for this heinous crime before the courts."
Chief Superintendent John O'Reilly from the Cavan/ Monaghan Garda Division said: "Today's arrests demonstrate An Garda Siochana's ongoing commitment, working closely with the Police Service of Northern Ireland, and is a further significant step to ensuring that the perpetrators of this vicious attack will be brought to justice."
Last November a key suspect in the crime died of a heart attack while being arrested by police in Buxton, Derbyshire.
Cyril McGuinness (54), known as 'Dublin Jimmy', was a serial criminal and was believed to have been in hiding in England at the time.
In an emotional interview on the BBC last year Mr Lunney spoke candidly about surviving the ordeal.
"I felt that I was going to die on the road," he said.
"I felt I was trying to get to the light in the house, a lot of things went through my mind and I almost gave up.
"I gave up a few times and came back again, thought about the kids and thought about Bronagh (his wife), and surviving."
He added: "There's a lot of police activity and hopefully that will bring fruition, but it needs to get to the person who is paying for this."
The joint PSNI-Garda investigation continues.
Police have urged anyone with information "no matter how insignificant it appears" to contact the PSNI at Enniskillen on the non-emergency number 101, quoting reference 1748 for September 9, 2019.
Information can also be passed on anonymously to Crimestoppers on 0800 555 111, or to Cavan Garda Station on 049 436 8800.