The Minister for Defence, Mr Michael Smith TD, has said there is "no such thing as, if you like, complete military neutrality".
Speaking on the US military's use of Shannon airport and the possibility arms are being transported through the airport, the Minister said as far as he is aware, the US military has not transgressed Irish aviation law.
But he added: "I suppose you can never give absolute guarantees on everything."
The Green Party said the Minister "in an unguarded moment" had "let the cat out of the bag that Irish neutrality is a sham".
Party chairman Mr John Gormley TD said the Minister's comments would embarrass the Government. He added the comments proved the Green Party's claim that Irish neutrality had been eroded over a long period.
"Ordinary people know that we are now assisting the American war effort," he said.
The Labour Party spokesman on foreign affairs, Mr Michael D. Higgins TD, said it was "outrageous" of the Minister to suggest US aircraft using Shannon were being continually monitored by the Irish authorities.
Mr Smith said yesterday US aircraft using Shannon "are very carefully watched and monitored, and no transgressions of these acts as far as I know have taken place."
However, the Government previously has said it takes at face value assurances from the US military that no arms are being shipped through Shannon.
Briefing reporters after he signed a new €60 million defence deal at Casement Aerodrome in Baldonnel yesterday, the Minister said Ireland and other nations had a responsibility to play their part in the war against terrorism. He added Ireland's economic dependency on the US adds to our responsibility.
"We have a lot of contact with America, a lot of business with America, we have a lot of people in this country who depend on America for their jobs," he said.
"So there is mutual interest in dealing with terrorism and co-operation because the big threats to ordinary innocent people today are coming from small groups of extremely dangerous people who are, and can be, involved in biological threats and suicide bombings."
When asked if US military use of Shannon compromised neutrality, he said: "There's no such thing as, if you like, complete military neutrality. And there certainly can be no neutrality when it comes to the threat of ordinary innocent people."
