Yesterday's confirmation by the Minister for Transport, Mr Brennan, that the "open skies" policy would eventually spell the end for the Shannon stopover was met with a mixture of concern and scepticism from Opposition politicians.
Labour's transport spokeswoman, Ms Róisín Shortall, said Mr Brennan was "engaging in wishful thinking" on the issue.
"We need a stronger commitment in terms of getting new routes to ensure any loss of business is counteracted," Ms Shortall said.
She said she was not against the policy per se as it opened up potential for all of the airports, but unless something was done soon the "vast bulk" of business would go to Dublin.
The future for the Shannon region lay in providing faster rail access to Shannon Airport, according to Ms Shortall.
Fine Gael's transport spokesman, Mr Pat Breen, said any diminution of Shannon's current status would have catastrophic consequences for the west of Ireland.
"I am seriously concerned that Shannon Airport will be become the sacrificial lamb," he said.
"In a recent Dáil debate on the issue of Shannon, the Minister reminded me that he is a west of Ireland man. Well, now is the time for him to prove his western pedigree and stand up and speak up for Shannon."
Sinn Féin's transport spokesman, Mr Seán Crowe, said he was very concerned about the implications for the workers directly employed at Shannon.
"It would again appear that the west and its people are going to be sacrificed in the interests of the market," he said.
Concern at the Minister's support for the EU "open skies" policy was expressed by a western lobby group, Atlantic Technology Corridor (ATC), yesterday. Its chief executive, Mr Pat Shanahan, expressed "serious disappointment" at the Minister's decision.
"We are seriously concerned at the erosion of Shannon Airport by the delay of the Minister's decision in relation to the airport's direction and the impact this has on business confidence in the region," he said.
Trade union leaders criticised the proposal and said they would campaign for its retention.
Both SIPTU and the Technical, Engineering and Electrical Union (TEEU) were sharply critical of Mr Brennan.
SIPTU's vice-president, Mr Jack O'Connor, said retention of a regular transatlantic service with increased frequency from Shannon was critical to the maintenance of thousands of jobs in the west and south-west.
His union did not regard the stopover as "some kind of sacred cow", he stressed. "But we will continue to oppose its removal in the absence of any well thought-out coherent plan, properly funded and supported by Government, to guarantee increased transatlantic traffic through the airport."
Accusing the Minister of having "blind confidence" in the free market, he said there was no evidence of any such plan at present.
Mr Arthur Hall, national secretary of the TEEU, said that without the retention of a year-round transatlantic service, US companies would not continue to invest in the mid-west region and thousands of tourists would bypass the area.
The TEEU would join with other unions in Aer Rianta in opposing Mr Brennan's "pie in the sky" plans, said Mr Hall.