Flights between Ireland and the US may be required to carry armed gardaí or security personnel, under new American anti-terrorist measures.
Under the regulations, which became effective yesterday, US authorities may now insist that specific flights scheduled to land in or take off from the US carry armed officers.
It comes after British authorities confirmed that armed sky marshals would be deployed on some transatlantic flights.
A spokesman for the Department of Transport said that, although it was highly unlikely to happen, sky marshals from other countries on aircraft landing in Ireland would also need permission from the Irish authorities.
In contrast to the US and Britain but like most other European countries, Ireland has decided not to introduce armed sky marshals on flights originating in this State.
However, the new regulations in the US may override this on transatlantic flights if they are deemed to be at risk from terrorist attack.
The US is on a heightened security alert due to the threat of terrorist attacks, with intelligence sources suggesting civilian aircraft could be used again.
The directive, which became effective yesterday, requires armed law-enforcement officers to be on board a flight, if requested by the US authorities.
"We are asking international air carriers to take this protective action as part of our ongoing effort to make air travel safe for Americans and visitors alike," said Mr Tom Ridge, the US Homeland Security Secretary.
Homeland Security spokesman Mr Dennis Murphy said the move will apply to specific flights "based on specific information".
Mr Murphy explained: "We will then notify the carrier that, based on information we received, we require a law-enforcement officer to be on the plane.
"What we are saying here is we expect this level of co-operation from all nations. This step is in case we might not get that same level of co-operation that we've received thus far from our closest allies. We anticipate the same level of co-operation from all air carriers that fly to and out of the US."
A spokesman for the Department of Transport said that, although under ongoing review, civil aviation security was not necessary on Irish flights.
The British authorities have also been in contact about their plans for air marshals, and have given a commitment to seek clearance before introducing them on British-Irish routes.
The Irish Airline Pilots Association has said it is opposed to armed personnel on civilian flights.
"We do not accept that it is necessary to introduce armed officials on to aircraft," said Capt Conor Nolan, director of safety. "The aircraft should be the last place for security measures. They should have taken place on the ground before take-off."
The Irish pilots' position mirrors that of their counterparts in Britain, who reacted angrily to government plans there to place armed marshals on some international flights, saying guns could fall into the hands of would-be attackers.
The only Irish airline likely to be affected by the new US regulations is Aer Lingus, which flies to four American cities.
According to Irish security sources, the use of Ireland as a launching pad for a terrorist attack remains remote.