A 23-year-old man has been arrested in connection with the murder of Gary Douch in Mountjoy prison this morning.
The man, who is also an inmate at the prison, was arrested just before 4pm this afternoon.
He is being detained under Section 4 of the Criminal Justice Act and can be questioned for up to 12 hours.
The Director General of the Prison Service, Brian Purcell, has described murder as a terrible tragedy.
Speaking to RTÉ News, Mr Purcell rejected suggestions that overcrowding in the jail had contributed to the death.
Mr Douch, who was 21 and from Dublin, was attacked and beaten in a cell he was sharing with up to seven other prisoners.
Independent inquiry
The Minister for Justice, Michael McDowell, has announced the establishment of an independent inquiry into the circumstances surrounding the death.
Mr Douch had earlier asked to be put on protection as he feared for his safety. He is understood to have been serving a three-year sentence.
The inquiry will be headed by a former senior civil servant, Michael Mellett, who is currently working with the Independent Monitoring Commission.
Mr McDowell says the report will be published in due course.
Meanwhile, the Prison Officers' Association has strongly criticised Mr McDowell for the ongoing overcrowding problem at Mountjoy Prison.
The association says it wrote to the Prison Service last month highlighting the dangerous, unhealthy and intolerable situation for staff and inmates at the Dublin jail.
It said the fact that Mr McDowell closed down three prisons contributed to the serious overcrowding problem at Mountjoy, where there have been a number of serious and violent incidents before today's murder.
