The IRA has admitted killing the only prison officer murdered in the Republic during the Troubles.

Thirty years on from the attack on Brian Stack, Sinn Féin leader Gerry Adams has expressed his "regret" to the Stack family over the killing.

Austin and Oliver Stack, the former Portlaoise chief prison officer's sons, held talks with Mr Adams and a former IRA commander in recent weeks as part of a long campaign to secure a confession.

The Stack family said that despite three separate Garda investigations, no individual or organisation has been found responsible for the prison officer’s murder.

In a statement, the family said a case review by the Serious Crime Review Team between 2007 and 2008 uncovered major flaws in the original investigation.

The case was then passed to the National Bureau of Criminal Investigation, but the Stack family said it has become “frustrated at what we can only describe as the uncooperative nature on the part of the current NBCI investigation team”.

The family said it was in this context that they met with Mr Adams and asked him to help them gain answers and “some measure of closure”.

Mr Adams said he accompanied the two brothers to a meeting with a former IRA commander, who admitted the IRA shot their father in the back of the neck on 25 March, 1983.

In a statement, the commander said Mr Stack was murdered as a reaction to the “brutal” prison regime in Portlaoise.

He said the killing had not been authorised by the IRA leadership and this was why the IRA denied any involvement.

The statement continued: “Some years later, when the Army Council discovered that its volunteers had shot Prison Officer Brian Stack, the volunteer responsible for the instruction was disciplined”.

Mr Stack had been crossing a busy Dublin street after leaving a boxing contest at the National Stadium when he was shot.

"I want to pay tribute to the Stack family - to Sheila Stack and her sons, Austin, Kieran and Oliver," said Mr Adams.

"On behalf of Sinn Féin I extend my regret at the killing of Brian.

"I hope that these recent developments will help them achieve the closure they have sought for 30 years."

Brian Stack was left paralysed and brain-damaged from the shooting and died from his injuries 18 months later aged 47.

His family said today that while the acknowledgement from the IRA commander brought some closure they were still left with unanswered questions.

The Garda Press office says a detective Garda Superintendant has been appointed to liaise with the family of Brian Stack.

In a statement they say the investigation into his death, and the liaison with the family, is ongoing and as such "it would be inappropriate to comment any further."

Minister for Justice Alan Shatter said Brian Stack's murder was not only "a tragedy for his family but an attack on the foundations of this State."

Mr Shatter said while he is aware of the frustrations of the Stack family over the investigation, he said gardaí faced challenges "when persons will provide information to others but not cooperate with their inquiries."