Gardai ruled out using bullet-proof glass in vehicles carrying Pope John Paul II on a Papal visit a month after one of the bloodiest days of the Troubles.

A security risk assessment in September 1979 claimed no measures could protect the Pontiff if "desperate persons" planned an assassination.

The Pope visited Ireland from September 29 to October 1 1979 - just over a month after the IRA blew up Lord Mountbatten in Mullaghmore and massacred 18 British soldiers in Warrenpoint.

Officers repeatedly highlighted the potential risk of a sniper attack but opted not to use bullet-proof glass. Despite this, they still insisted all reasonable and practical security measures were being taken.

"The unanimous opinion of the officers involved in co-ordinating operational and security matters was that bullet-proof glass should not be used," the document stated.

It later added: "It must be borne in mind, however, that if desperate persons plan to kill the Pope, then no security measures which the gardai could implement would guarantee the Pope's safety.

"There is no such thing as absolute security."

The document, marked secret and released by the National Archives under the 30-year rule, emphasised potential security risks posed to the Pope at several locations - a sniper attack was listed at three of four points.

These included the air journey from Rome with Aer Lingus, his arrival at Dublin Airport and the Nunciature and the open-air Mass in Phoenix Park.

In each case the risks and suitable precautions were highlighted, with an attack by sniper as a threat listed in three of the four locations. The document stated all possible sniper positions would be manned by gardai and army personnel.