Aer Lingus is operating a normal flight schedule today despite the continuing dispute with cabin crew over rosters.
The trade union Impact has claimed that the quarrel could be costing the airline more than €400,000 a day.
The company has in recent days been hiring in aircraft and crews from other carriers, including Ryanair, as part of a contingency plan to allow it to operate its schedule.
About 140 members of cabin crew, who are represented by Impact, have been removed from the payroll by Aer Lingus as part of the dispute over rosters.
On Friday, the airline said it would give cabin crew who had been removed from the payroll a further opportunity to agree to work the controversial new rosters.
It said if they did not agree to do so, they would enter a disciplinary process which could see them sacked.
Letters setting out this position are being sent out on a phased basis to the 140 cabin crew who have been taken off the payroll.
The first group of staff who received such letters are scheduled to attend "investigative meetings" with Aer Lingus management tomorrow.
Separately, Impact has asked the Data Protection Commissioner to carry out an investigation into Aer Lingus on the basis that the company had provided personal details, including mobile phone numbers, to a courier company it hired to deliver the letters to its staff.
In a statement yesterday, Impact called on Aer Lingus to reveal how much it was costing it to hire in aircraft and crews while it had sent home its own staff and left expensive assets lying idle. A spokesman for Aer Lingus declined to comment on the bills the company was incurring in organising its contingency arrangement for maintaining its flight schedule.
The company has said the new rosters are essential to give effect to a cost-saving plan which involves increasing flying hours for cabin crew to 850 a year.
The union says the new rosters are highly onerous and in breach of the cost-saving agreement.
Impact says the mounting costs include:
Hiring aircraft and crews, which the union estimates could be in the region of €40,000 for a European round trip and over €250,000 for a transatlantic round trip. The union estimates that at least 10 aircraft a day are being hired;
Refunds to passengers whose flights were cancelled;
The opportunity cost of idle Aer Lingus aircraft, which are expensive capital assets;
The cost of hiring outside staff to conduct disciplinary hearings against cabin crew;
The salary costs of pilots left with no work because hired aircraft have their own crew;
Running newspaper advertisements.