Fresh doubts have been cast over the testimony of top FBI informer David Rupert in the Omagh civil action, which is sitting in Dublin.
It has emerged that a senior Garda attempted to block a statement that would have eroded credibility in Rupert's testimony against alleged Real IRA leader Michael McKevitt.
The civil action by six families is against five men they believe are responsible for the Omagh bomb in August 1998 which killed 29 people, including a woman pregnant with twins.
An internal M15 memo has revealed that Garda Chief Supt Dermot Jennings wanted to remove the report because it undermined Rupert, who provided damaging testimony against McKevitt five years ago.
The document expressed fears that if Jennings disputed that he told Rupert he was "only being interested in illegal activity in the [26 Counties]" it would make Rupert appear to be an untrustworthy source.
Rupert's evidence was paramount in the conviction of Mr McKevitt, who was found guilty and jailed for 20 years on the newly described charge of "directing terrorism".
Chief Supt Jennings, promoted to Assistant Commissioner in 2001, was not called to give evidence at McKevitt's trial, and is not listed to appear before the Omagh case.
Michael O'Higgins SC, for McKevitt, told Dublin District Court that Rupert and Chief Supt Jennings had numerous meetings in 1997.
Earlier, the trial heard that Rupert was, by his own admission, a "drug smuggler, people smuggler, brothel keeper and confidence trickster".
Despite a past which included multiple bankruptcies, being investigated for fraud and owing hundreds of thousands in unpaid taxes, the Garda never bothered to investigate the past of David Rupert, counsel for McKevitt told the trial.
Michael O'Higgins SC said David Rupert was the "beginning, middle and end" of the case against McKevitt.
He said Mr Rupert had a long history of deception and greed and that he fleeced and ripped off everybody he came into contact with, yet the State had never asked questions of his credibility as a witness.
Mr Rupert had asked for $2 million to infiltrate and destroy the Real IRA, Mr O'Higgins also stated.
Det Insp Diarmuid O'Sullivan, one of the principal investigators in the original trial, said that if the Garda had got involved in investigating Rupert's past, it would be regarded as "interference".
The court also heard that it was "obvious" that Garda officers had memorised undated statements for the hearing.
The accuracy of testimonies given by two Gardai, who each maintained they saw the accused before and after a meeting in a house, were questioned by counsel for the defence.
Kieran Vaughan QC pointed to admissions to the court that at no time did any officer consider video recording or photographing the meeting, that they did not log times or car registrations.
Mr Vaughan also said that reports and subsequent statements from all three Gardai were worded and phrased the same and had not been dated, suggesting they were written together.
The Gardai denied the claims.
The court case is currently being heard in Dublin, which entails evidence from a British Crown case being heard in the 26 Counties for the first time.
The legal teams are expected to ask Mr Justice Morgan, who has presided over the case in Belfast Crown Court but has no formal judicial powers in Dublin, for an extension of hearings in the 26 Counties.