The much-vaunted pre-clearance facility at Dublin Airport is understood to be at risk.
It is believed there are issues with the service offered to passengers by the US Customs and Border Protection (CBP).
Passengers travelling from Dublin to the US can currently save two hours of queuing at immigration there by pre-clearing in Ireland.
Dublin is the only European country offering the facility, which lets passengers avoid long waits and collect their bags once they land.
However, Irish officials have warned Transport Minister Shane Ross that while pre-clearance "operates very well", there have been difficulties with "CBP manpower resources".
The Irish Transport Department said that while all airlines that wanted to use the service were pre-cleared in 2015, the facility was now under threat.
"With increasing passenger numbers there is a risk that all flights will not pre-clear in 2016 unless permanent additional resources are provided by US CBP," a spokesman added.
A spokeswoman for the US CBP acknowledged queries last night but did not respond before our print deadline.
Business and travel organisations here have warned a reduction in the pre-clearance service would have major consequences for Ireland.