Dublin's attractiveness as a transatlantic hub is under threat with plans to offer pre-clearance facilities for US travellers at 10 global airports.
Dublin and Shannon are currently the only airports in Europe that currently have the pre-inspection facilities, which are hosted by US Customs and Border Protection (CBP).
The value of the Irish airports for transatlantic traffic was a key reason for IAG's €1.4bn plan to buy Aer Lingus.
But now the US Secretary of Homeland Security, Jeh Johnson, has announced that the United States is entering into negotiations with 10 foreign airports that could eventually be bases for the coveted pre-clearance system.
The airports selected include Heathrow and Manchester.
Pre-clearance means that passengers can clear US customs and security before leaving a territory, allowing them to enter the United States classed as domestic flyers.
That enables them to avoid what are often lengthy immigration queues at US airports when they touch down there.
At Dublin Airport, it is thought that about 30 US personnel are involved in administering the service.
David Holohan, the head of research with stockbroking firm Merrion Capital, said the advent of CBP at Manchester could pose a threat.
"Manchester is an attractive regional airport out of the UK and on that basis it will be a threat. The number of passengers going through it is quite material," he said.
Manchester handles about 23 million passengers a year compared to the 21.7 million at Dublin in 2014.
British Airways-owned IAG already uses facilities at Shannon for an all business-class service it operates from Heathrow to New York's JFK. Passengers on those flights can pre-clear US customs and security at Shannon before continuing their journey to the United States.
Manchester Airport also unveiled a planned £1bn (€1.37bn) investment programme yesterday.
It aims to lure more airlines and add new routes to Asia and the east and west coasts of the United States.
The Department of Homeland Security called for expressions of interest from airports last November and received more than two dozen applications.
Ironically it cited the success of the Dublin operation in the application document.
IAG chief executive Willie Walsh told financial analysts last week after the Government approved the sale of its 25.1pc Aer Lingus stake to IAG, that he did not expect that Heathrow would be able to successfully implement a US customs and pre-clearance facility.
At Terminal Two in Dublin, the building was specially designed for the purpose of installing the US pre-clearance system.
The other airports being considered for the service include Brussels Zaventem, and Amsterdam's Schipol airport. The 10 airports vying for the service handled a total of 20 million passengers last year who were flying to the US.