The family of prison officer Brian Stack, who was shot dead by the IRA over 30 years ago, has accused one section of An Garda Síochána of frustrating the inquiry into his murder by withholding information from investigating officers.
Brian Stack, a former chief prison officer at Portlaoise prison was shot in Dublin in March 1983. He died 18 months later.
Mr Stack's son, Austin, said he was told that the information is "very significant" and claims it has been kept on a file in Garda Headquarters since 1990 to protect the identity of an informant.
RTÉ News understands it relates to the identities of the gunman, a motorcyclist and the person alleged to have sanctioned the murder.
Mr Stack said the family has sought a meeting with the Garda Commissioner for the past three years but have not been successful.
He has now called on Taoiseach Leo Varadkar to intervene to enable the investigation team to have full access to all garda files.
A Government spokesman said: "The Taoiseach believes the murder of Brian Stack by the Provisional IRA was a cowardly and vicious act.
"The Taoiseach wants the perpetrators of this heinous crime to be brought to Justice. However, the Taoiseach cannot comment on the details of an ongoing criminal investigation.
"It is the Taoiseach's understanding that the Department of Justice has been assured by the Gardaí that there is no attempt to impede the investigation."
Minister for Justice Charlie Flanagan has said he has been assured by garda authorities that there is no credence in any suggestion that there is an impediment on the part of the gardaí.
Minister Flanagan said he has known the Stack family for many years and would be willing to meet them to discuss their concerns.
He said Mr Stack's murder was one of the most heinous and brutal acts of The Troubles.
Speaking on RTÉ’s Today with Sean O'Rourke, Austin Stack said that in November 2015, a retired garda, who had worked in counter terrorism, approached him with information about his father's murder.
The garda said he had received the information from an IRA informant and had put the information on the file in 1990.
Mr Stack said that his family spoke to gardaí in 2016 to see if they were aware of this information.
He described this meeting as "quite robust" and said "unparliamentary language" was used towards them and they were told that their information was inaccurate and incorrect. Since then, he said, his mother has refused to meet gardaí.
In March 2017, Mr Stack said that gardaí came back and said that the information was correct and had been found.
However, in December 2017, the Stack family were told by the investigating team that it was unable to access the information and they did not know the name of the informant involved.
In a statement, gardaí said that it could not comment on the details of ongoing investigations and neither does it "comment on remarks by third-parties".
It said the team leading the investigation into the death were "satisfied that they have been provided with all the relevant information they require from within An Garda Síochána" and that it is getting cooperation from "all relevant sections" within the gardaí to pursue all lines of inquiry.
"An Garda Síochána maintains regular contact with the Stack family for the purpose of keeping them informed of any relevant developments in this investigation" and that gardaí are determined to bring the investigation into Brian Stack's death "to a successful conclusion."
