The Republic of Ireland's justice minister Frances Fitzgerald has ordered a full investigation into reports that a garda leaked information to a senior IRA figure
It is understood that the communication was detected by British security service MI5 who were monitoring the phone of the dissident. They contacted the PSNI who in turn informed An Garda Siochana.
The officer sent information to the man about two suspected terrorists who had been arrested and were being questioned in a Leinster station where the garda was based.
In a statement the Department of Justice 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.
"The minister has asked the Interim Garda Commissioner Noirin O'Sullivan for a full report on this matter."
The incident happened within the last nine months.
It is understood that the information passed on by the former garda included the names of the detained suspects and how long they were being questioned by detectives.
The garda did not have 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.
Senior gardai became aware of the situation when they were contacted by the PSNI. The garda later resigned from the force and is also the subject of investigations into separate serious offences.
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 Continuity IRA member who previously served a lengthy jail sentence for bomb making offences.
Meanwhile, a high-profile solicitor has contacted the Taoiseach and Justice Minister about suspected garda collusion in a series of other murders.
John McBurney, who represented the family of murdered RUC Chief Supt Harry Breen at the Smithwick Tribunal, said the revelations are "of major significance".
"I welcome the fact that the Justice Minister is taking this extremely seriously as she should," Mr McBurney said.