Four people have been arrested as part of a cross-border investigation into the abduction and torture of Quinn Industrial Holdings (QIH) director Kevin Lunney in September 2019.
Mr Lunney was left by the side of a road in County Cavan after a two and a half hour long attack.
Two men aged 23 and 61, and a woman aged 61, were arrested by the PSNI in County Fermanagh on Thursday morning.
A man in his 30s was detained by gardai (Irish police) in County Cavan.
A joint investigation team involving detectives on both sides of the Irish border was established in November 2019.
The three suspects arrested in Northern Ireland are accused of conspiracy to kidnap, conspiracy to cause grievous bodily harm with intent and assisting offenders.
Gardai said they arrested a man in relation to the abduction and assault for an offence contrary to section 72 Criminal Justice Act 2006.
Detective Chief Inspector Julie Mullan from PSNI's Criminal Investigation Branch said: "The arrests today show the close working partnership between the PSNI and An Garda Síochána and our determination to bring those responsible for this heinous crime before the courts."
Chief Superintendent John O'Reilly, Cavan/Monaghan Garda Division said: "Today's arrests demonstrate An Garda Síochána'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."