A prisoner was taken to hospital after he was held hostage by fellow inmates at Mountjoy jail in Dublin last night.
The victim was treated for non-life threatening head injuries. Gardai and the Irish Prison Service have launched an investigation into the disturbances at about 6.30pm.
A complaint about the visitor system at the Dublin centre was to blame, the Prison Service said.
A spokesman for the Service said: "There was an incident in Mountjoy. Five prisoners held another prisoner against his will in a multi-occupancy cell.
"The stated reason behind the incident was a complaint about the booked visitor system in Mountjoy. The situation was resolved peacefully.
"One prisoner has been taken to the Mater Hospital after receiving injuries around his head, the injuries are not life-threatening."
The inmates, who were on recreation, were immediately returned to their cells and the prison locked down. The matter was resolved by about 9pm.
PA