Aer Lingus suffered a fall in passengers carried in September of 2.9% over the same month last year.
Numbers were down to 989,000 year-on-year, with a 4.4% drop in short-haul numbers to 881,000.
The short-haul load factor declined by 3.7 percentage points to 77.7%, with capacity cut by 4.6%.
Long-haul performed better in the month with an increase of 11.3% to 108,000.
The long-haul load factor was down by two percentage points to 88.7% as capacity rose by 14.7%.