A former MI5 surveillance officer has said he made illegal incursions into the Republic tracking dissident republicans and meeting with double agents.
Codenamed Robert Acott, the former spook claims he was involved in breaching internationally treaties by crossing the border without the permission of the Irish foreign office.
Speaking to reporter Nick Hopkins for a BBC Newsnight investigation, the former intelligence officer said that after suffering a mental breakdown as a result of the trauma of the job he was abandoned by his former employees.
He was later dismissed for misconduct after a file was found in a garden shed of his home but he told the Newsnight team that the file was not classified and had no security status.
Acott says he travelled all over the world for MI5, following suspects, including alleged IRA members to Europe and the Middle East on a passport with a false identity.
Among those he claimed to have been following was Diarmuid O'Neill who was shot dead in London in 1996 during a police raid on the hotel where he and two other IRA men were staying.
The only IRA member to be shot dead by police in England, Amnesty International at the time called for an investigation into the shooting.
Acott claimed he was following the suspect for months prior to his shooting, "Operation Tinnitus was a huge job, that resulted in Diarmuid O'Neill being shot dead in a lodgings house"
"I followed him day in day out for absolute months and I know it seems a bit strange to say but when he was actually killed I felt a bit sad for him," he said.
Acott also went to the Republic to provide covert help to MI5 handlers who had meetings with IRA double agents without the knowledge of the Irish government.
The former agent says his health began to deteriorate in 2006 when he started to have nightmares and panic attacks and was drinking to help cope with the pressure.
"I know I was in a mess at the time. But the service didn't want to deal with that. Once they realised I had health problems, I think they just wanted rid of me", he said.
MI5 refused to comment on the claims but in a letter sent to a relative of Accott said: "It is important to reiterate that this Department is not responsible for his health problems, nor do his problems stem from his time working here."
Fianna Fail, justice spokesman Senator Denis O'Donovan said he would look at the claims raised by programme with a view to raising the matter with the justice minister Alan Shatter.
