The former detective sergeant who is one of the three former officers at the centre of the Smithwick Tribunal has started to give evidence.
Owen Corrigan said that he had no involvement in the murders of Chief Supt Harry Breen and Supt Bob Buchanan on 20 March 1989.
It has been claimed that Mr Corrigan passed information to the IRA, which gave them time to set up the ambush in which the RUC men were killed just minutes after leaving a meeting in Dundalk Garda Station.
Today he said that allegation was "a shameful, monstrous and terrible lie." Mr Corrigan said he had served the State loyally despite the impact it had on him and his family.
He said that he welcomed the setting up of the tribunal and the opportunity to give evidence to clear his name. Mr Corrigan said it was with deep regret that his wife, Sheila, had died before he had the chance to vindicate his name.
The witness said that during the Troubles, Dundalk "was no oasis of peace."
There were 300 people who were either subversives or had links to subversives based in the area at the time. There were also loyalist murder squads operating as well.
He spoke about the three IRA members who were shot by the SAS on an operation in Gibraltar and how they used to travel through Dundalk on their way to prepare for their attack on British soldiers.
Mr Corrigan also said that he could not continue to work in Dundalk Garda Station because of the way he was being treated.
He wanted to stay working but, he said, he felt he had no choice.
When he put in for retirement, the then Head of Crime and Security Branch Noel Conroy came to see him and asked the witness to work for him.
He had an operation under way to try and recover the Beit Paintings, which had been stolen from Russborough House by Martin Cahill.
Mr Corrigan said, however, that he withdrew from the operation after his wife expressed concerns to him.
Justin Dillon, counsel for the tribunal, put it to Mr Corrigan that he had been given an option to transfer to the Special Detective Unit in Dublin rather than retire from the force.
However, the witness said that "dubious" things had been alleged against him and he was coming near retirement age.
Mr Dillion put it to him that he then went on "sick leave on a pretty grand scale" and that during this time he was negotiating to buy a pub and another premises.
Mr Corrigan said he was certified ill at the time and that he was not the first nor would he be the last garda to go on sick leave.
Had things been reasonably normal in Dundalk he would have survived there, but he was not going to be ill-treated by anyone.
Asked what he meant, he said that when your supporting power base collapses "you are very vulnerable."
Earlier, the tribunal had heard from Jimmy Spratt, DUP MLA for South Belast.
He worked as the team leader for the close protection unit for former RUC Chief Constable Sir Jack Hermon.
He disputed evidence given to the tribunal that some gardaí from Dundalk were on first name terms with Mr Hermon as they drove escort cars on his visits south of the border.
Mr Spratt said this never happened and that invariably the escort unmarked garda car was provided and staffed by members of the Crime and Security Unit based in Garda Headquarters.
He said there was one occasion when that did not happen and a garda car from Dundalk was provided.
Mr Hermon, he said, told them to lose the tail which they tried to do and instead of crossing at Kileen, just north of Dundalk, they took a completely different route through Monaghan.
The witness also said that he overheard conversations in the car when he was there with Mr Hermon with senior Special Branch officers.
The name of Mr Corrigan was mentioned but Mr Spratt refused to go into detail in the circumstances saying he was prevented from doing so by the Official Secrets Act.
Mr Corrigan told the tribunal that when there was a change in senior officers in the Dundalk station, he was never approached by any of them asking for information.
"I didn't feel my expertise was being appreciated," he said.
The witness went on to say he did not approach them but that it was up to the senior officers to come to him as they were his supervisors.
A confidential memo from his Chief Supt John Nolan to Asst Commissioner O'Dea regarding Mr Corrigan, dated from August 1989, was put to the witness.
It said that he had "opted out" from policing but Mr Corrigan said he would not accept that. He insisted he was treated very badly after all the service he had given to Dundalk.
"My records of achievement is there for all to see," declared Mr Corrigan.
"I was ignored because of a change in personnel at the top."
He was also asked about going missing on the night that John McAnulty was abducted, tortured and killed by the IRA.
Mr Corrigan said he took grave exception to anyone questioning his loyalty to the gardaí.
He said he was involved in many "very, very dangerous missions, meeting very, very dangerous people."
The people who criticised him were far away from all of this while it was going on.
"Those making the complaints were not there when I needed them," he said.
The former detective sergeant said Dundalk was described as ''El Paso'' during the time of the Troubles and it was not wrong to describe it as such.
He said he found it deeply offensive that he as a member of a police force, would be accused on being involved in the deaths of other police officers.
This afternoon, the tribunal has been hearing about the positive recommendations made about Mr Corrigan while he served in the force.
He was described as "an excellent detective" who had a long interest in tackling subversives and that "it would be difficult to find a more dedicated and effective interrogator".
He also said he ceased applying for promotion to the rank of Detective Inspector after three failed attempts.