Two men and a woman have been arrested as part of the investigation into the kidnapping and torture of Quinn Industrial Holdings director Kevin Lunney.
They are being held at garda stations in Kells, Cavan and Monaghan.
One man is aged in his 20s while the other is in his 40s, while the woman is aged in her 50s.
Other searches were carried out in Cavan, Longford and Dublin.
Gardaí and the PSNI have established a joint team to investigate a campaign of intimidation, including death threats, targeting the QIH directors.
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.
Last week the chief suspect in the investigation, Cyril McGuiness, died during a police raid in the UK.