A major security alert was sparked when a rogue garda officer passed on key information to a senior IRA figure in Northern Ireland, it can be revealed.
The garda passed on information about two dissident Republican suspects who were being questioned in the Leinster station where the officer worked.
It is understood the information involved the names of the detained suspects and how long they were being questioned for by detectives.
The garda sent a text message to the senior IRA figure, who is based in Northern Ireland.
This was picked up by intelligence agencies monitoring phone traffic.
Senior gardai became aware of the situation when they were contacted by the PSNI - who had been tipped off by MI5.
They had been using the British state intelligence agency to monitor all calls and text messages to and from the IRA man's phone.
Investigations by the PSNI later established that the person who gave the information to the IRA figure was in fact a garda.
The incident happened in a Leinster garda station within the past year.
It has since emerged that the garda has very close links to a man who was arrested in relation to the 1998 Omagh bombing, in which 29 people were murdered including a woman pregnant with twins.
And in recent times the former garda has been spotted in the company of a middle-aged Continuity IRA member who previously served a lengthy jail sentence for bomb making.
"This garda's behaviour has caused a huge amount of shock to their colleagues," a source told the Herald newspaper.
"What was happening is highly unusual and is in no way any reflection on the people who were stationed with the garda."
It is understood no further action will be taken in the case, because the officer has since resigned from the force and is the subject of investigations into separate serious offences.
The now-retired garda joined the force within the last decade at a time when background checks were sometimes not as scrupulous as they are now.
It is not believed the officer had access to information about what was being said during the questioning of the suspects, which took place after their arrest by armed officers under the Offences Against the State Act.
In a completely unrelated case, the Smithwick Tribunal found last year that an IRA mole in Dundalk Garda Station tipped off an IRA hit squad and this led to the murders of two of senior RUC men in March, 1989.