The system of recording conversations used physical tapes which were rotated and taped over repeatedly except in exceptional circumstances. This method of recording means that material taped would only be available until it was taped over — often within weeks in the busier stations.

“You might have a tape changed every second week in the busy stations,” a source familiar with the technology used said, “and then that tape would be rotated and maybe three months later would be used again and overwritten”.

The news is expected to ease fears that a whole raft of criminal convictions could potentially be overturned following the announcement from Government last week that the recording regime had been in place since the 1980s.

It also ensures that a huge volume of work in listening to the tapes will not now have to be undertaken as part of the Commission of Inquiry announced by the Government last Tuesday.

With the actual conversations taped rendered inaccessible, the possibility of court challenges in a large number of cases prior to 2008 would most likely collapse. A regime based on digital web-based equipment was installed in 2008.

The recording in the main Garda stations was undertaken by a company called Dictaphone, which has since gone into liquidation. The company held the contract for installing and maintaining the equipment “for a very long period of time”, believed to be up to 20 years. The system involved use of what was called a “guardian box”, which was installed in a dedicated room in the stations. One person, usually but not exclusively a garda, was trained on use of the equipment and how to listen back to the tapes.

“Usually it was used for 999 calls,” the source says. “But sometimes if a garda thought he got something wrong in taking down a note from somebody who rang in, he might ask to listen back to it so he can correct things.”

The only tapes that were retained were those due to be used as evidence in court, such as an occasion when a member of the public rang in a crime or incident. This would then be handed over to the DPP or state solicitor and might not be returned to the station. It is possible some of the material taped over could be retrieved, but this is unlikely in most cases.

The news is expected to ease some pressure in government circles over the potential for wholesale challenges in the courts.

Meanwhile, the Irish Examiner has learned senior counsel Sean Guerin has been in contact with Garda whistleblower Sergeant Maurice McCabe. Mr Guerin was appointed by the Government six weeks ago to look into a dossier compiled by Sgt McCabe containing allegations of serious malpractice in criminal investigations. Mr Guerin is due to report by Easter on whether a full commission of inquiry should be conducted into the affair.