Aer Lingus has said that 40 new jobs announced last July by the airline will not now go ahead.
The company said that 40 more cabin crew based out of Shannon could face redeployment or redundancy.
The IMPACT trade union is to ballot cabin crew for industrial action in response to management's move to outsource staff on transatlantic services.
Last July, Aer Lingus announced it was almost doubling the frequency of transatlantic services from Shannon next year, but using a smaller Boeing 757 aircraft.
The company plans to lease the aircraft from external provider ASL Aviation.
The company said it would be a crew of four - something the company says the Irish Aviation Authority had approved.
But the trade union IMPACT said the number was an issue that had to be negotiated and agreed but the company refused to engage on the issue.
Aer Lingus says it was informed by the trade union that it would not operate transatlantic flights with four crew.
In a communication to staff this evening about what Chief Executive Christoph Mueller said: "IMPACT's own goal is a lost opportunity."