Robert O’Connor (34) was rushed to the Mater hospital after fellow inmates launched the brutal attack in the man's cell in the Dublin prison.
He was fighting for his life in ICU after sustaining a serious brain injury before his death was confirmed this morning.
Senior sources say that such were the horrific injuries that O’Connor suffered in the attack on Friday evening that he was never expected to survive.
The ventilator that was keeping him alive at Dublin’s Mater Hospital was switched off last night and detectives from Mountjoy Garda Station are now preparing to launch a murder investigation.
All four inmates who entered the victim’s cell on Friday evening before O’Connor was savagely beaten were identified on CCTV and have been separated from all other prisoners.
Gardaí have not questioned them yet and they are all expected to be moved to other prisons away from each other.
“The investigation team is in no rush in this matter because none of those involved in this are going anywhere soon apart from the prison system and gardaí have a lot of time to build up the evidence of what actually happened,” a senior source said.
“There is a large number of theories being investigated about what happened including whether the victim was targeted over an alleged sexual assault that happened before he got locked up, a row that he got in the jail system, a drugs debt he may owe or whether associates of a Finglas criminal who he was allegedly on the way to target when he was caught with a gun last year are behind it.
“This is a long list but every possible motive will be scrutinised and in this case gardaí are in no rush whatsoever before the suspects are questioned,” the source explained.
O'Connor had only been sentenced two days before the attack for six-and-a-half years after admitting to the possession of a semi-automatic pistol in Finglas, Dublin, last October.
His previous convictions include drug offences, stealing cars, criminal damage, theft and road traffic offences.
The victim was not a major player in any of the prominent criminal gangs but he did suffer from serious drug issues.
Of the suspects identified, one is linked to the Kinahan cartel while another is serving a lengthy sentence for an aggravated burglary.
All are from the capital’s northside and are considered extremely violent criminals.
In a statement this morning, the Irish Prison Service confirmed the news.
“The Irish Prison Service can confirm that the prisoner who was seriously injured in an incident in Mountjoy Prison on Friday 29th July 2022 has died.”
“We extend our deepest sympathies to his family and friends.”
The incident continues to be investigated by the IPS and the Gardai.