Four people have been arrested as part of allegations of witness intimidation in the Aaron Brady murder trial.
The Garda National Bureau of Criminal Investigation's also investigating allegations of perverting the course of justice and the unlawful use of mobile phones in prisons.
It's in relation to the recent murder trial of Aaron Brady.
Mr Brady of Crossmaglen, County Armagh was convicted recently of the capital murder of Detective Garda Adrian Donohoe.
The four are being questioned at Mountjoy, Ballymun, Bridewell and Raheny Garda Stations.