A mother and son are being questioned in connection with the abduction and torture of Quinn Industrial Holdings executive Kevin Lunney.
They were arrested along with another man earlier this morning. They are being questioned at separate Garda stations.
A Garda spokesperson said the the woman, aged in her 50s, is being held under the Offences Against The State Act.
The two men - one in his 20s and another in his 40s - are being held under the Criminal Justice Act.
Mr Lunney was kidnapped and brutally assaulted by a group of masked men in September.
The attack was the most serious act in a campaign of intimidation against executives at the company that has been ongoing for years.
Last week, gardaí and the PSNI launched a cross-border taskforce to investigate the attack and the wider campaign – which included a number of death threats.
The task-force was announced following a series of raids on both sides of the border and in England.
Gardaí searched 12 locations south of the border while the PSNI searched five locations in the North.
Meanwhile, British police were searching the home in Derbyshire.
Police said one of the main suspects in the investigation, Irishman Cyril McGuiness, died during the raid in Derbyshire.
The 58-year-old was taken ill after police carried out a search warrant at an address in Buxton where he was staying.
This morning, Gardaí appealed to anyone with any information "no matter how insignificant it appears, on this vicious criminal attack" to contact them.
They are also looking for information any other criminal activities which have taken place in the Cavan/Fermanagh border area over recent years.