An Garda Síochána has rejected claims by the family of murdered chief prison officer Brian Stack that a section in the force frustrated the inquiry into his death by failing to pass on “very significant” information from an IRA informant to the investigation team.

Mr Stack was a prison officer in Portlaoise prison and was shot in the neck in Dublin on March 25th, 1983. He died 18 months later.

His son Austin Stack said he had been told in November 2015 by a retired senior garda that “significant information which had come from an IRA informant” was passed to gardaí in 1990 and that this information contained details of those responsible for his father’s murder.

Mr Stack said the retired member had told him he had contacted senior gardaí in 2013 and informed them about the file.

Mr Stack claimed the information had been on a file in Garda headquarters since 1990. He called on Taoiseach Leo Varadkar to intervene.

While the Taoiseach did not comment on the matter at a press briefing at Government Buildings on Wednesday, Minister for Justice Charlie Flanagan said he was “concerned at reports that there would be any form of impediment regarding this investigation”.

However, “I’m assured by the Garda authorities this morning that there is no credence in any suggestion that there is any impediment on the part of the gardaí themselves who are treating this investigation as a live investigation.”

Or Mr Stack’s death, the Laois TD said: “His callous murder was one of the most heinous and brutal acts of the Troubles. There is a live garda investigation here and I’m very anxious that anybody who is in any position to provide any information or evidence on the murder of Brian stack would do so.”

Asked if he would meet with the Stack family, Mr Flanagan said he had known the family for decades and would be happy to meet with them.

In a statement earlier, Austin Stack said: “My father was murdered because he was protecting the State, and I am asking the Taoiseach to demand answers from the Garda commissioner over this and instruct him to allow the investigation team have full access to all files.

“The family met with the investigation team and an assistant commissioner in January 2016 and made them aware of this information.”

He said gardaí got “quite irate” with the family at the meeting when they revealed what they knew and that their information had been “rubbished”.

Austin Stack (left) and Oliver Stack, sons of Brian Stack, pictured in 2013. Photograph: Aidan Crawley/The Irish Times

He said Brian Stack’s widow, Sheila, had refused to meet gardaí since then.

“At a meeting in March 2017 the gardaí apologised to the family for the manner in which our information had been dismissed, and they admitted that they had found the file in another section and that it contained very significant information,” Mr Stack claimed.

“The family were disappointed to be told at a recent meeting that the investigation had not progressed and the new information had not been accessed fully.”

Mr Stack said it had become obvious to the family “that one section of the gardaí is frustrating the investigation in order to protect an informant who they say they have a duty of care to.

“The gardaí also have a duty of care to my mother and our family, and we feel that the role my father played in protecting the State is being undermined by a lack of a proper investigation.”

Mr Stack said the family had requested a meeting with the Garda commissioner three years ago and reiterated this request in early December but had not yet received a response.

An Garda Síochána said that in order to protect investigations, it did not comment on the detail of ongoing investigations and that it did not comment on remarks by third parties.

“The investigators leading the investigation relating to the death of Mr Brian Stack are satisfied that they have been provided with all the relevant information they require from within An Garda Síochána,” a Garda spokesman said.

“The investigation team is receiving the co-operation of all relevant sections within An Garda Síochána in order to pursue all possible lines of enquiry.

“An Garda Síochána maintains regular contact with the Stack family for the purpose of keeping them informed of any relevant developments in this investigation.”

The spokesman said the force was “determined to bring the investigation into the death of Brian Stack to a successful conclusion”.

Mr Stack’s wife and sons Austin and Oliver met former taoiseach Enda Kenny and former tánaiste Frances Fitzgerald in January last year to discuss their concerns.

The family claimed at the time that Sinn Féin president Gerry Adams had information about Brian Stack’s murder, and they urged him to make that known to gardaí.

The Stack brothers met Mr Adams in 2013. The Sinn Féin leader took them in a blacked-out van to meet a senior IRA figure who told them the organisation was responsible for their father’s murder.

The family want Mr Adams to give gardaí the identities of the person they met and the person who drove the van.