Aer Lingus said on Friday that it carried 634,000 passengers in December, 4.6 percent more than in the same month of 2005, but its planes were not quite as full as it added extra routes.
The former state airline said its load factor was 72.6 percent last month compared with 73.8 percent in December 2005.
On short-haul flights the load factor dipped to 66.6 percent from 67.3 percent while on long-haul the factor dropped to 80.4 percent of capacity from 81.7 percent.
Overall, the carrier's load factor fell to 77.6 percent in the 12 months to the end of December from 81.4 percent in 2005 although the number of passengers carried last year rose 7.3 percent to 8.6 million as it added new routes.