Gardaí have begun an investigation and say they are following a definite line of inquiry in connection with the death of a man at Cloverhill Prison in Dublin overnight.
Emergency services were alerted shortly after midnight following an altercation between two prisoners.
A man in his 30s was treated at the scene, but attempts to resuscitate him were not successful and he was pronounced dead a short time later.
His body has been taken to the city morgue, where a post-mortem examination is due to take place this afternoon.
A second man sustained minor injuries during the incident, which is also being investigated by the Inspector of Prisons and the Irish Prison Service.
In a statement, the IPS said that all practicable measures were taken by prison management and staff to ensure the safety of those in custody.
But it added that no prison system could be entirely free of violence, and it said that violence between prisoners did happen.
Additional reporting Paul Reynolds