The British attorney-general has given the green light for the lid to be lifted on the investigation into the murder of two senior RUC officers minutes after they left a garda station in 1989, and also into the brutal killing by the IRA of Louth farmer Tom Oliver because he was a "garda informer".
The go-ahead from London is likely to lead to the naming of the mole inside the gardai who told the IRA that Louth farmer Tom Oliver was telling police about the activities of terror groups operating on the Cooley peninsula.
Mr Oliver, a farmer from the Carlingford area of Co Louth, was brutally murdered by the IRA -- a crime that led to widespread protests throughout the area.
In the last few days, the lawyer for the Smithwick Tribunal in Dublin has informed solicitors representing potential key witnesses, including former the British Army agent Kevin Fulton, that it has received an undertaking from the attorney-general of England, Wales and Northern Ireland regarding evidence before the tribunal.
The letter to legal representatives says that an undertaking has been given that no evidence given by witnesses or made in statements will be used in evidence against them in any criminal proceedings taking place in England, Wales or Northern Ireland.
The undertaking was given after months of negotiations between lawyers working for the tribunal and the attorney- general's office in London.
One of the key potential witnesses, former soldier Fulton, has already made an initial statement to the tribunal but has refused to agree to testify without immunity from prosecution.
It is understood that Fulton has told the tribunal that he can connect a then-serving garda officer with the killings of Chief Superintendent Harry Breen and Superintendent Bob Buchanan in March 1989, just minutes after the two senior RUC officers had left Dundalk garda station after discussing the crime empire of IRA leader Thomas 'Slab' Murphy and other IRA activity in south Armagh.
But it is also understood that Fulton has crucial details about the betrayal of Tom Oliver by a member of the gardai. The father of seven from Riverstown, near Dundalk, was found shot dead in Belleek in County Armagh in July 1991 after he was abducted for a second time by the IRA and questioned about being a garda agent.
Fulton is understood to have been involved in the first abduction of Oliver which resulted in his release unharmed by the IRA. But the IRA then obtained a tape recording of the farmer talking to his garda handler from a public phone which the IRA had bugged.
Fulton, who was working in France at the time Oliver was murdered, has given the tribunal's lawyers details of the first abduction of the farmer and how the IRA was alerted to the farmer's role as a garda agent.
Its also understood that he identified the role of another British Army agent, Freddie Scappaticci, in the first interrogation of Tom Oliver.
The undertaking by Britain's attorney-general to give immunity to witnesses to the Smithwick Tribunal is a major boost for the inquiry, which has been stalled for almost two years.
The tribunal wrote to lawyers before Easter and is now hoping to take detailed statements from potential witnesses including Fulton. Other witnesses could include former agent handlers whose testimony could confirm the role of Scappaticci.
Lagan Valley MP Jeffrey Donaldson welcomed the legal undertaking. "This is a good development which hopefully will enable the tribunal to move on to take evidence in public soon.
"My concern is that agents like Kevin Fulton will be absolutely assured that they can reveal the secrets they know without fearing any subsequent legal consequence. That is the essential undertaking they need," he said.