Aer Lingus suffered a drop in carryings last month as the impact of strike action took its toll.
The Irish airline was hit by 24 hours of industrial action last Friday in a dispute with cabin crew over rosters.
May passenger carryings were down by 0.8% to 924,000 against the same month last year.
Short-haul numbers dropped by 3.5% to 795,000 while long-haul carryings rose by more than 20% to 129,000.
The overall load factor was down by 1.1 percentage points to 78.7%.
The Impact union which represents cabin crew has warned there would be further industrial action, probably running over two days, if there was no progress in talks with the airline.
Cabin crew are demanding a more structured roster arrangement which would see them work for five days and then be off for three days. They say this fixed roster arrangement is currently in place for pilots at the airline.
It is understood the airline has said it would look at the introduction of such a roster arrangement if it could be put in place on a cost-neutral basis, the Irish Times reported.