Total Aer Lingus passenger numbers declined by 3.8% last month in contrast to a 20% rise by Irish rival Ryanair in December.
Aer Lingus carryings were down to 705,000 in the month due to an 8.2% drop in short-haul numbers to 517,000.
Long-haul numbers were up by 20% to 102,000 while Aer Lingus regional operations saw passengers rise by 1.2% to 86,000.
The overall load factor was down by 1.8 percentage points to 72.1%.
Ryanair handled more than 6 million passengers in December, a rise of one million over the same month in 2013.
EasyJet carried 3.2% more passengers in the month to reach 4.6 million with an improved load factor of 88.4%.
The UK budget airline’s annual carryings for the year to December were up by 6.5% to more than 65 million.