A former Provisional IRA sympathiser and his girlfriend have been arrested by gardai in connection with the cross-border investigation into the murder of prison officer David Black.
The two were detained by armed officers from the garda Special Branch, while a third arrest was made by the PSNI in Tyrone.
Gardai detained the couple on the northside of Dublin yesterday morning on suspicion of being involved in the planning of the murder of Mr Black, who was shot dead as he drove to work at Maghaberry jail last November 1.
A recently formed alliance of dissident terror groups including the Dublin-based faction of the Real IRA later confirmed it was responsible.
The car used by the gunmen in the attack on the father of two had a Dublin registration and had not been stolen. The abandoned dark blue Toyota Camry was bought shortly before the murder, and gardai have been trying to trace its history.
The two Dublin suspects, aged 44 and 31, were being questioned last night at Ballymun garda station about playing a logistical role in the lead-up to the murder.
The day after the murder gardai arrested a 29-year-old man in Carrigallen, Co Leitrim, on suspicion of withholding information.
He was later released without charge, while the PSNI also set free two suspects, including prominent republican Colin Duffy.
Mr Black was ambushed as he drove on a motorway in Co Armagh. The terrorists later claimed the attack was connected to a dispute over conditions at Maghaberry.
Meanwhile, the Republic's Justice Minister Alan Shatter last night described the Continuity IRA (CIRA) as "criminal terrorists" and said he did not believe the words "dissident" or "republican" should be associated with them.
He was speaking in a Dail debate about yesterday's disclosure in the Irish Independent that gardai had foiled a CIRA plot to murder a member of the British army while he was spending Christmas at home in Limerick.
Gardai uncovered the murder plot during an intelligence gathering operation and found out when and where the terrorists intended to kill their target.
They also became aware that a gun had been sourced and a hitman had been chosen.
Mr Shatter told the Dail there was an ongoing garda operation in Limerick aimed at halting the activities of a certain group, but would not comment on a specific case.
He said gardai remained active in their efforts to counteract paramilitary gangs and he congratulated the force for its successes.
Fianna Fail justice spokes-man Niall Collins said the foiled attack was a "very chilling thought for all to comprehend and take on board".
He added: "What we read this morning in the Irish Independent had potentially devastating consequences had it succeeded."
He believed it would have inflicted extreme reputational damage on the Limerick area, the Mid West and the country in terms of business, foreign direct investment and tourism.