A murder investigation is under way after the fatal attack on a 34-year-old inmate at Mountjoy Prison last week.

Robert O'Connor from Snowdrop Walk, Darndale in Dublin, died in hospital last night. A post-mortem has now concluded.

He was attacked in his cell on Friday evening and sustained serious head injuries. He was taken to the Mater Hospital and was being treated in intensive care.

O'Connor had been in custody since last October, but was sentenced for firearms offences two days before the fatal attack.

The Prison Service expressed its sympathies to his family and said it would carry out investigations along with the Inspector of Prisons to assist gardaí.

The attack on O'Connor only lasted a number of seconds and prison officers were quickly on the scene, but he had already sustained serious injuries.

Gardaí said they are keeping an open mind on the motive and are investigating whether the attack could be linked to a drug debt, an earlier row within the prison or the foiled gun attack for which O'Connor had been jailed.

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An incident room has been set up and a family liaison officer has been appointed.

They have interviewed a number of people in the prison, but believe prisoners are unlikely to co-operate.

They have secured CCTV footage of the area and the clothes they believe the suspects wore, while four men have been isolated within the prison.

A forensic examination of the landing and cell has also been completed.

'Unusual' death

Executive Director of the Irish Penal Reform Trust Saoirse Brady said that violent deaths in prison are a very rare occurrence.

"While deaths do occur in custody, it's very unusual for it to be a violent death like this," she said.

Speaking on RTÉ's News at One, Ms Brady said: "In this particular case, without going into the details, it's been reported that there wasn't a perceived threat."

She said that there were currently more than 500 prisoners on what is deemed a "restricted regime" for protection reasons and this involves remaining in a cell for upwards of 19 hours a day.

Figures from April showed that 575 prisoners were in this type of protective custody, 563 of these had requested it.

She said: "Where any death happens in custody, there are a number of people that need to be notified and the Inspector of Prisons is one of them, and they will carry out an investigation into the circumstances surrounding the death.

"They'll look at what care was provided by the Irish Prison Service.

"They'll examine the operational methods used, what policies are in place and they also ensure that the prisoner's family will have an opportunity to raise any concerns that they might have.

"And then also their investigation report will help the coroner's examination into the death, but obviously, it's a matter for the coroner to determine the cause of death."

Ms Brady said that very often when the Office of the Inspector of Prisons carries out an investigation of this kind, it can be years before it is published after being sent to the Minister for Justice.

She said: "There is very often a delay in the Minister for Justice deciding to publish that report, so it could be months or even years before that is published.

"We would like to see a more timely publication of those reports because they make recommendations to the Irish Prison Service, they put in place an action plan and the Irish Prison Service has to respond to that as well and outline what actions it will take to ensure that lessons are learned and that similar incidents don't happen in the future."

The Irish Penal Reform Trust has called for an action plan to implemented in relation to deaths in custody to prevent another tragic situation happening.

Inmate was serving six-year sentence

O'Connor had been sentenced to six-and-a-half years in prison.

He had pleaded guilty after he was caught with a loaded semi-automatic pistol and a magazine with three more rounds in Dublin last year.

A threat assessment was carried out in Mountjoy and he was initially moved from one part of the prison to another after an issue arose.

O'Connor expressed no fears for his safety and was not put in protective custody.