A major security alert was sparked when a rogue officer passed on key information from a garda station to a senior IRA figure, it can be revealed.
The Herald has learned that the garda passed on information about two dissident Republican suspects that were being questioned in the Leinster station where the officer worked.
Justice Minister Frances Fitzgerald has described the incident as 'disturbing' and said she is establishing the facts of the situation as a matter of urgency.
The Minister has asked the Interim Garda Commissioner for a full report into the incident.
In a statement this evening the department said:
"It would be completely unacceptable if information of any kind were to be passed by a member of An Garda Siochana to members of unlawful organisations".
"It is understood that An Garda Siochana is aware from its own intelligence and enquiries of suggestions that a member of the force passed on the names of persons who had been arrested on a particular occasion.
"The member in question has since resigned and is currently under criminal investigation in relation to this and an unrelated matter."
It is understood that the information involved the names of the detained suspects and how long they were being questioned for by detectives.
It has emerged that the garda sent a text message to a senior IRA figure based in the North.
This was then 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.
However, it is not believed the garda 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.
Garda headquarters were notified of the incident but it is understood that no further action will be taken in the case.
The reason for this is the garda has since resigned from the force and is the subject of investigations into separate serious offences.
The now retired officer joined the force within the last decade at a time when background checks were sometimes not as scrupulous as they are now.
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 that 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.
"What was happening is highly unusual and is in no way any reflection on the people who were stationed with the garda."
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.