The US government has blamed an administrative error after a US plane armed with a fixed weapon landed at Shannon Airport without permission.
Details of the unauthorised landing were revealed by Tánaiste and Minister for Foreign Affairs Eamon Gilmore in response to a parliamentary question by Labour TD Derek Nolan.
In a written reply to Mr Nolan, Mr Gilmore said he was aware that a US plane armed with a fixed weapon landed at Shannon Airport in the period prior to September 2013.
He said the case was raised at the highest possible level with the US Embassy.
An official there said it was an isolated incident that had taken place as a result of an administrative error, he said.
The minister said assurances were given that steps had been taken to prevent any recurrence.
Under Irish law, permission is normally granted to military aircraft landing at Irish airports, provided the aircraft are unarmed and carry no arms, ammunition or explosives.
In the period to September 2013, 366 such landings occurred.
All but 30 of those involved US military planes.