Aer Lingus today said its passenger numbers for September were 927,000, down 3.4% on September 2009.
Long haul passengers were 86,000, equal to September 2009, while short haul passengers were 841,000, a 3.8% decrease on last year, the airline said.
The airline attributed the fall in part to a reduction in capacity.
"Aer Lingus significantly reduced its long haul capacity at the end of 2009 and made a smaller reduction to short haul capacity earlier this year," the company said in a statement.
"These changes were made to achieve a better match between capacity and the still declining market demand in the core market of Ireland, and to remove loss-making routes.
"As a result, total September 2010 capacity was 7.6% lower than last year."
The airline said its load factor - the amount of passengers flown as a percentage of the amount of seats available - for September was up 3.5 points on last year to 81.0%.
Long haul load factor was 81.8%, an increase of 10.2 points on September 2009, with capacity decreasing by 15.5%, the airline said.
Short haul load factor was 80.6%, a decrease of 0.2 points on September 2009, with capacity decreasing by 3.3%.