Four men have been arrested in connection with a garda investigation into allegations of witness intimidation connected to the trial of Aaron Brady.

The 29-year-old from New Road in Crossmaglen in Co Armagh was found guilty of the murder of Detective Garda Adrian Donohoe and is due to be sentenced next week.

The presiding judge referred during the trial to one incident of intimidation as "the most outrageous" contempt of court he had seen.

The four are being detained at separate Dublin garda stations.

The Garda National Bureau of Criminal Investigation has been investigating allegations of witness intimidation, perverting the course of justice and the unlawful use of mobile phones in the prison environment relating to the trial of Aaron Brady.

He was found to be one of the gunmen at the Lordship Credit Union on the night of 25 January 2013 who shot dead the Detective Garda during an armed robbery. 

It was the longest murder trial in the history of the State but during it there were repeated claims that witnesses were afraid to give evidence because of intimidation. 

Five witnesses did not testify - one did not come forward after a video of him giving a statement about Aaron Brady to the gardaí was circulated on social media with text calling him a "rat" and accusing him of "touting" on his friends. 

Mr Justice Michael White called it "the most outrageous" contempt of court he had seen in his career. 

Today the gardaí arrested four men on suspicion of intimidating witnesses and perverting the course of justice 

The four are aged 46, 38, 29 and 28. All four were arrested in Dublin, two of them were arrested in the prisons where they are serving sentences for other offences.

They are being detained at four separate Dublin garda stations - Raheny, Ballymun, the Bridewell and Mountjoy - and can be questioned for up to 24 hours.