The arrest of Aaron Brady has been described as "central" to the investigation into witness intimidation during the Adrian Donohoe murder trial.
Garda killer Brady (29) was taken from his jail cell in Portlaoise Prison's A wing yesterday morning and brought to the nearby garda station.
His arrest is part of a major probe by the National Bureau of Criminal Investigation (NBCI) into the interference of witnesses, perversion of the course of justice and the use of mobile phones behind bars related to the seven-month murder trial.
On Thursday, a man known to Brady, aged in his 50s, was detained as part of the same probe in Co Monaghan before being released without charge.
This individual has regularly been visiting the convicted garda killer and was detained after travelling from the North into the south.
The arrests are the sixth and seventh made as part of the inquiry, with one source describing the arrest of Brady as "central" to the investigation.
Throughout the murder trial gardaí and the prosecution repeatedly raised their concerns about witnesses being interfered with and contacted by Brady's family.
In May, during an in-camera hearing from which members of the public were banned, it emerged that a video had been leaked and shared online of a witness giving a statement on camera to gardaí .
The video had been circulated widely on several social media platforms branding the witness a "rat" and "a tout" while alleging they had made the statement to secure a green card.
Presiding judge Mr Justice Michael White described it as the most outrageous contempt of court he had ever come across.
Within hours an investigation was launched by members of the Serious Crime Review Team.
A number of searches were carried out on jail cells and properties in Dublin within weeks of the witness statement video circulating.
One cell searched was that of Dean Byrne (28), a convicted burglar with over 120 previous convictions. A piece of paper with phone numbers written on it was recovered.
The Mountjoy cell of Glen Holland, linked to the Kinahan cartel, was also raided.
Both men were arrested as part of the inquiry but were later released without charge.