Mountjoy escape: A Department of Justice memorandum for the Government in November 1973 admitted, in effect, that little could be done to prevent a repeat of the spectacular helicopter escape from Mountjoy Prison the previous month, organised by the Provisional IRA.
Three leading republicans, Séamus Twomey, Kevin Mallon and J.B. O'Hagan, escaped in a helicopter which landed in the exercise yard of Mountjoy on October 31st that year. The helicopter was originally hired by a man posing as an American film producer but an IRA unit ordered the pilot to fly to Mountjoy. Stunned prison guards reportedly thought it was a surprise ministerial visit.
Responding to criticism of security precautions, the Department of Justice memo warned: "Shooting a helicopter down might result in the deaths of many people (depending on where it fell) - it could, for instance, fall on one of the prison officers' houses just outside the prison wall."
A helicopter escape "certainly was foreseeable", as were many other possibilities. "Weapons could be dropped from a helicopter or light plane or even dropped over the wall of the prison by means of a mobile crane. A bazooka attack could be launched against the prison gates."
But preventing these possibilities could give rise to undesirable side-effects. "The possibility of a direct rescue by helicopter such as occurred on October 31st could be largely eliminated by denying any outdoor recreation to the prisoners concerned. This would no doubt lead to trouble - hunger-strikes, etc."
The memo stated: "While the range of what can be foreseen is almost infinite, the principle of the tolerable limits of risk (expressed or otherwise) comes into play at some stage."