Ryanair recorded five million passengers last month, a 2% rise on the corresponding month in 2009, according to data from the airline released yesterday.
The figure represents the number of booked seats, not necessarily the number who actually flew with the airline.
December snowfalls across Ireland and the UK, as well as other European countries, forced the cancellation of a large number of flights. The carrier said its load factor slipped one percentage point in December to 81%, compared to the corresponding month in 2009. Ryanair recorded 72.7m passengers in 2010 and a load factor of 82%.
The passenger numbers include nearly 1.5m who were booked on flights that were cancelled due to the Icelandic volcanic eruption during the year.
Ryanair also announced the launch of a new twice-weekly service between Cork and Milan Bergamo yesterday. It will start on June 2.
Earlier this week it emerged that the Office of Fair Trading (OFT) is to proceed with a probe into Ryanair's near-30% holding in Aer Lingus after it determined that it remains "in time" to examine the acquisition of the stake in the former state-owned carrier.
Ryanair controversially withdrew from Belfast in October 2010 following a long-running row over an runway extension at George Best Belfast City Airport.