Gardai investigating the kidnap and torture of businessman Kevin Lunney have arrested four men.
The Garda announced on Thursday morning that two suspects, both aged 38, had been detained in the Dublin region.
Later in the day, the force confirmed two further arrests.
A 25-year-old man was detained in the Dublin area while a 66-year-old man was arrested in Co Cavan.
All four suspects were being held at separate Garda stations on Thursday afternoon for questioning.
In September, Quinn Industrial Holdings (QIH) director Mr Lunney, 50, was abducted outside his home near Derrylin, Co Fermanagh, and taken to a horsebox across the border, where he was savagely beaten.
His attackers broke his leg, sliced his fingernails and face with a Stanley knife, carved QIH on his chest, and doused the father of six in bleach.
Two men and a woman arrested last week in relation to the attack were later released without charge.
A convicted criminal considered a key suspect died earlier this month during a police raid in England.
Known gangster and smuggler Cyril McGuinness suffered a heart attack while Derbyshire Police were searching a house where he was staying in the Buxton area.
Mr Lunney spoke publicly about his ordeal for the first time earlier this month in a powerful TV interview.
The attack on Mr Lunney, during which his assailants demanded his resignation, was the most serious of a five-year campaign of intimidation targeting the companies and directors that now control the business portfolio that was built up by fallen tycoon Sean Quinn, once Ireland’s richest man.
A sinister element in the community in the Fermanagh/Cavan border area, where the companies are based, continues to vent anger at the demise of Mr Quinn’s empire.
The Quinn family has consistently condemned and distanced itself from those attacking the new owners.
Mr Lunney’s interview has created an intense public and political focus on the intimidation campaign.
Taoiseach Leo Varadkar, Garda Commissioner Drew Harris and PSNI Chief Constable Simon Byrne have all met Mr Lunney and his fellow directors in the period since.