John Lonergan was speaking after Robert O’Connor (34) died from catastrophic head injuries in the early hours of yesterday morning in the Mater Hospital after being attacked in his cell last Friday.
It has emerged that Mr O’Connor had been moved to a new wing for his own protection after he had been beaten with a kettle just two days before he was attacked.
However, Mr Lonergan told RTE’s Morning Ireland that if someone was determined to attack another prisoner then they would get the opportunity.
“The fundamental purpose and the top priority of the prison service is the safe custody of those in prison and that includes the personal safety of prisoners,” he said.
“Any individual person who is being committed to prison should expect and has a right to expect that his or her safety is guaranteed in the prison.
“But the reality is of course that there's always a risk factor and unfortunately on rare occasions people do suffer serious injury and on a very small number of occasions, lose their lives.”
He agreed that while killings are rare, attacks and beatings are “pretty common”.
“This is something that has happened over the last 25 years. During the previous hundred years very few serious assaults took place in prison. Certainly you had no organised crime in prison, you hadn't gangs or the drug feuds and the gangland feuds that you have nowadays.
“And they have been the biggest single contributory factor to the level of violence. There's been a massive increase in violence in prisons over the last 20 years.
"This is despite a huge amount of resources being put into security and to preventative measures with highly trained staff and a huge amount of emphasis on reducing the opportunities that prisoners would have to arm themselves with weapons.
“We have over 400 prisoners in what's called protection at the moment and that's a tenth of the overall prison population. Protection means that they are taken out of the mainstream and they have a more reduced and confined regime and their safety is certainly enhanced.
“But it also means that there is an ongoing challenge even amongst that 400 to recreate them and to have any type of positive regime.
“We in Ireland operate on a free association basis which means that prisoners generally can mix and socialise within the prisons. But free association brings with it the risk that if an individual prisoner or a group of prisoners set out to damage another prisoner that opportunity will be there.
“You have to remember as well that on exercise for instance you can have 100 to 150 prisoners walking around a yard and you can imagine the opportunities that would be there for prisoners if they were so determined to carry out attacks like that.”
However, Mr Lonergan stressed that it was important to reassure the families of prisoners who “might be anxious and worried about the well-being of their own particular individual family members in prison”.
“For the vast numbers of people that are not involved in the drug culture our prisons are very safe,” he added.
“This is very much a gangland drug feud environment and anybody who is involved in that type of activity on the outside it's almost inevitable that that particular activity will continue on the inside.
“Because the gangs our imprisoned eventually and they establish themselves back in prison, and in all prisons, not just in Mountjoy, and not just in Ireland either.”
Meanwhile, jail sources have revealed that a number of prisoners have been disciplined for the earlier attack on Mr O’Connor and gardaí are investigating if the assault on the prison’s A-Wing is linked to his murder on the C2 landing.
A prominent line in the inquiry is whether O’Connor was targeted in a revenge attack after he ordered that another inmate be slashed in revenge for an alleged assault on one of the close associates of the murder victim on the outside.
Senior sources said that this is one of just a number of motive theories being examined.
Last Friday O’Connor was rushed to the Mater Hospital where he was kept alive on a ventilator after being diagnosed with a severe brain injury which had been originally expected to be turned off on Monday.
His life support was switched off in the early hours of yesterday morning but gardaí have been treating the case as a murder investigation since Friday because of the injuries that the victim received.
O’Connor had been on remand in custody in Cloverhill Prison since last October but was moved to Mountjoy in February after picking up a six-month sentence for a separate offence.
Last Wednesday, he pleaded guilty at Dublin Circuit Criminal Court to possession of a semi-automatic pistol in Finglas on October 13, 2021.