Six Ryanair flights to and from Dublin are among several affected today.
These include services to Berlin, Glasgow and Krakow.
The carrier has published the full list of flight cancellations for the next five weeks.
It says these cancellations have been allocated to its bigger base airports, and routes with multiple daily frequencies.
The airline says the cancellations are due to a backlog in its annual leave for staff.
The company is moving its holiday year from the current system of April to March to a calendar year of January to December from January 2018.
It says the backlog must be cleared before December 31st this year.
Affected customers will be contacted directly and offered alternative flights or full refunds.