Mr Gilmore made the admission in response to a question from party colleague and Galway West TD Derek Nolan, and said the issue was raised at the “highest possible level” with the US authorities here.
Although Mr Nolan’s question didn’t specify any particular incident, he did ask Mr Gilmore if all military landings “complied with normal conditions”.
These conditions include that the military aircraft landing here are unarmed, carry no arms, ammunition, or explosives, do not engage in intelligence gathering, and that the flights in question do not form any part of military exercises or operations.
Mr Gilmore said: “For the period to the 30 Sept 2013, my department granted permission for the landing of 366 foreign military aircraft at Shannon Airport.
“Permission was granted in all cases subject to the normal conditions that apply to landings at Irish airports by foreign military aircraft.”
He admitted: “I am aware of one landing at Shannon Airport during the period in question in which a US military aircraft, contrary to notification, was found to be armed with a fixed weapon and did not, therefore, comply with the normal conditions that apply to foreign military aircraft overflying or landing in Ireland.
“This case was raised at the highest possible level with the US embassy. The embassy official explained that this isolated incident had taken place as a result of an administrative error, and assurances were given that steps had been taken to prevent any recurrence.”
No information on the type of “fixed weapon” has been provided. It could have been a machine gun, missile, bomb, or another armament.
A spokesman for Shannonwatch, a group that closely monitors US military aircraft movements at the airport, said: “We now know that the so-called assurances the US have given about not taking weapons through Shannon on military flights are worthless.
“For years now Shannonwatch have been asking for proper inspections of US military planes at Shannon, and this admission by Minister Gilmore vindicates our demands. Regular and routine inspections are now required until such time as the US military use of the airport is finally ended.”
It has also been confirmed that of the 366 military aircraft that landed at Shannon in the first eight months of the year, 336 were US planes while around 45,000 US military personnel passed through Shannon.