Four men have been arrested as part of the investigation into the abduction and torture of Quinn Industrial Holdings director Kevin Lunney.
Two of the men, both aged 38, were arrested during a planned operation in Dublin this morning.
They are being questioned at Ballymun and Finglas garda stations.
A 25-year-old man was also detained in the Dublin region today and he is being held at Irishtown Garda Station.
A 66-year-old man was arrested in Co Cavan and is being questioned at Monaghan Garda Station.
Earlier this month, two men and a woman were arrested as part of the inquiry. They were released without charge.
Mr Lunney was attacked on 17 September as he made his way home from work.
The 50-year-old father-of-six was found bleeding on a remote roadside in Co Cavan after being abducted outside his home in Derrylin, Co Fermanagh.
Mr Lunney told BBC Spotlight earlier this month that he was slashed with a knife, doused with bleach and branded by his captors.
He sustained serious injuries in the attack, including a broken leg and had the letters 'QIH' cut into his chest.
The chief suspect in the investigation, Cyril McGuinness, died during a police raid in the UK.
McGuinness, who was in his 50s, took ill when Derbyshire police searched his home in the Buxton area of the UK on 8 November.
Gardaí and the PSNI believe he was the main organiser of the kidnapping and attack.
An inquest into his death heard that McGuinness died from a suspected "cardiac event" after being handcuffed during his arrest.
Gardaí and the PSNI have established a joint team to investigate a campaign of intimidation, including death threats, targeting the QIH directors.