The mainly US planes, classed as civilian, had sought permission to fly over the country or refuel at Shannon but were rejected on advice from the Department of Foreign Affairs and forced to re-route.

No information has been revealed about the exact type of munitions believed to have been on board, a parliamentary inquiry into the use of the airport by US military was told.

Paschal Donohoe, the transport minister, said that aviation chiefs were acting on advice from diplomats who had inspected cargo manifests and read flight charts.

“The standard policy of the Department of Foreign Affairs is that it will not allow material carried through our airspace or munitions carried through our airspace that would be indiscriminate in nature,” the minister said.

The flights were refused entry to Irish airspace within 48 hours of their expected arrival based on what was written on the cargo manifest, where the plane was due to travel to or from or the international climate at the time, he added.

The Joint Committee on Public Service Oversight and Petitions — which is examining the use of Shannon by US military on the back of a petition brought to parliament by anti-war campaigner Ed Horgan — heard 65,952 US soldiers passed through the airport last year. The figure is 80% down from the peak and 4% of the total traffic.

It was also told 584 exemptions were granted last year for aircraft to land at Shannon with weapons on board compared to 1,495 in 2007.

Mr Donohoe said the vast majority of exemptions related to “munitions of war” for US soldiers. “This is more a function of geography than policy,” he said. “In many cases, the most direct flight path between points in North America and points in continental Europe passes through Irish airspace.

Some 90% of the exemptions involved US airlines chartered by Washington DC for troops carrying sidearms and the remainder were for aircraft carrying military hardware like ammunition, explosives and bombs, usually on cargo planes.

The committee was told weapons were carried unloaded and no ammunition was on board the flights.

Mr Donohoe claimed the Government relied on assurances from US officials over what guns and weapons were being carried on civilian aircraft and that extraordinary renditions were not taking place through Irish airspace or airports.