Published Friday, 28 August 2009
John Anthony Deery, 50, who was described as "vulnerable" had been in Maghaberry Prison, near Lisburn.
Medical staff tried to resuscitate him when they were called to his cell at the jail's healthcare facility on Thursday night, but he died on Friday afternoon in Belfast's Royal Victoria Hospital where he had been on a life support machine.
A statement issued on behalf of the Prison Service of Northern Ireland said in a separate incident at the same jail on Thursday night, another prisoner "with a history of serious illness" was transferred to the Lagan Valley hospital, also Lisburn, after he was found in his cell.
Mr Deery had been in custody within the past week.
A Prison Service spokesman was unavailable for comment.
The statement issued by the Northern Ireland Office said police, the prisons minister Paul Goggins and the Prisoner Ombudsman had begun an investigation into the circumstances surrounding the death.
The condition of the second prisoner is currently unknown.
He was taken by ambulance to hospital after he was found in Magheraberry's healthcare facility at 8.36pm on Thursday night, according to the statement.