The number of passengers booked to fly with Aer Lingus last month was 25.3pc lower than in December 2009 as cuts in the airline's capacity were exacerbated by bad weather.
And it said that 8pc of the passengers booked to fly with the carrier last month didn't actually travel with it.
The figures were released as the airline gears up to have all its London Heathrow-bound flights from Dublin operated out of the airport's new Terminal 2 facility from tomorrow.
Aer Lingus said 572,000 passengers were booked to fly on its aircraft during December, with the number of short-haul passengers falling 27.3pc to 504,000 and long-haul passenger numbers 6.8pc lower year-on-year at 68,000.
Its overall booked load factor was 2.7 percentage points better in December, at 74.6pc, than it had been in the corresponding month in 2009.
The short-haul booked load factor improved by 3.7 percentage points to 73.2pc for the month. That was on the back of an almost 32pc year-on-year cut in short-haul capacity, which fell due to planned cuts at Dublin, Cork and Gatwick, and the adverse weather conditions.
On long-haul services, the booked load factor slipped 0.2 percentage points to 76.6pc, with capacity shrinking by 6.6pc year-on-year.
Aer Lingus has operated 100 flights from T2 since it opened in November. Following the full transition of Heathrow flights to the terminal, the airline will begin transferring Gatwick flights to the facility.
It is expected that all Aer Lingus short-haul services will be transferred to T2 by the end of the month, with transatlantic flights being transitioned to T2 next month. Delta Airlines shifted its operations to T2 today.
Meanwhile, Air France said yesterday that snowfall in Europe and the US last month cost it €70m in lost revenue. Passengers carried in December fell 1.6pc to 5.4 million.
The airline had earlier estimated the snowfalls would cost it in the region of €25m to €35m.