A prisoner found hanged in his cell was the sixth suicide in 18 months at jails in Northern Ireland.
Staff at the top security Maghaberry Prison near Lisburn, Co Antrim, tried to revive Niall Oliver Leonard, 37, from Lisnaskea, Co Fermanagh, when they discovered him in a collapsed state at 2am,
Police and the Prisoner Ombudsman Pauline McCabe were notified and an inquest will be held later.
Mr Leonard, a widowed father-of-two, had been serving a three-month jail term for drink-driving charges after police stopped his Volkswagen car not far from his home in January. At the time he had been banned from driving.
The new Northern Ireland Prison Service director general Sue McAllister, who took up the job last week, said: "I would like to extend my sympathy to the family. This is a tragedy and my thoughts are with them at this difficult time."