The inmate strangled to death in his cell in Cloverhill Prison on on Saturday had only been in custody for a week before his death.
Mark Lawlor, 30s, was described as a "harmless soul" by heartbroken friends who say he had mental health issues.
Mark, who was facing theft charges, was well liked around the Sheriff St area and was known to many locals as 'Gerry'.
Pals described how 'Gerry' had family in the area but was known to sleep rough despite not being homeless.
One told Dublin Live: "Gerry is a harmless soul, he was well liked by the people of Sheriff st and they looked out for him.
"He's going to be missed in the area, he has family around but he just seemed to want to do his own thing.
"You'd always get a big hello from him when he passed by."
In a post on Facebook one friend said "anyone that knew him knows he wouldn't harm a fly."
Mark's body was discovered by prison staff shortly after midnight following reports of a vicious row with a fellow prisoner.
Reports suggest that warders were forced to pull a fellow prisoner off the him in his cell following an altercation but that he was already dead before staff got to him.
A man in his 50s has been arrested on suspicion of murder and is currently being detained in Clondalkin Garda Station.
A source told Dublin Live: "Inmates in the cell next to Lawlor's heard a commotion and notified warders. They opened the cell quickly but he was already dead and they had to pull the other inmate off him."
The cause of the fight is still unknown and it is suspected that Lawlor was strangled to death by his killer.
The source added: “There’s no rhyme or reason as to why it started yet.”
A spokesman from the Irish Prison Service (IPS) said that inmate safety is of the utmost importance.
The spokesman said: “All practicable measures are taken by prison Management and Staff to ensure the safety of those in custody.
“However, regrettably no prison system can be completely free of violence and inter prisoner assaults do occur.
“Investigations by An Garda Siochana, the Irish Prison Service and the office of the Inspector of Prisons are underway to establish the circumstances surrounding today’s incident.”
