Aer Lingus managers have briefed staff on details of the airline's cost-cutting plan that will see more than 670 workers losing their jobs.
Pay cuts are also to be implemented for staff earning more than €35,000 a year.
Aer Lingus said it will enter six weeks of consultations with unions and employees.
The company's new chief executive Christoph Mueller said he was open to constructive proposals, but he could not compromise on the amount of savings needed.
Unions have meanwhile warned that any attempt to impose the planned cuts without agreement could lead to industrial action.
ICTU General Secretary David Begg, who is a member of the Board of Aer Lingus, said there could be no question of imposing the plan on staff.
SIPTU has said it will not accept the measures and said it will take whatever steps are necessary to defend its members' interests.
In a statement, National Industrial Secretary Gerry McCormack said the union will convene a meeting of its National Shop Stewards Committee to consider the proposals.
IMPACT, which represents cabin crews, described the cutbacks as vicious.
Speaking on Morning Ireland, IMPACT Assistant General Secretary Christina Carney said the proposals were punitive.
The Irish Airline Pilots' Association said that while the proposals were as severe as expected, no public comment would be made until the contents had been considered.
In a statement to the markets this morning, Aer Lingus said it needs to reduce annual costs by almost €100m before 2011.
It also warned that it is not possible to deliver the required cost savings in line with the plan and within the required time-frame, it may have to cut more jobs in order to ensure the continued viability of the airline.
The airline also said that it will introduce banded reductions in wages for staff whose basic pay is more than €35,000.
The airline said it had developed a two-stage transformation plan to reduce costs and remove legacy work practices. It says this will enable it to compete more effectively with its peers who have significantly lower operating costs.
It said the changes implemented under the plan are expected to reduce the airline's operating costs, excluding fuel, by €97m a year. This is made up of staff cost savings of €74m and non-staff cost savings of €23m.
Aer Lingus said the first stage of the plan is expected to deliver significant change across the organisation, mainly in operational areas, although pay and staff numbers will also be reduced in support areas.
The second stage of the plan will focus on initiatives to deliver revenue growth, improved customer services and further cost savings through a series of business process improvements.
Passenger numbers up 4.1%
Meanwhile, Aer Lingus has reported that its passenger numbers in September rose by 4.1% to 960,000 compared to the same time last year.
Its short-haul passenger numbers increased by 7.4% to 874,000 but its long-haul numbers slumped 20.4% to 86,000.
It said its load factor, or how many seats it fills on each plane, rose by 2.5% to 77.5% as capacity fell by 7.6%.