The Commission for Aviation Regulation has said Ryanair will have to pay compensation to passengers whose flights have been cancelled with less than two weeks' notice.
A spokesperson said it is likely the airline may have to pay out hefty sums in compensation because of its decision to cancel 40-50 flights per day over the next six weeks.
The airline has so far given people notice of sometimes just hours and at most five days.
Eight Irish flights were cancelled for today, all of which were to or from Dublin Airport.
The flights cancelled were between Dublin and four destinations - Amsterdam, Frankfurt, Nantes and Santander.
The airline has said it is emailing all passengers whose flights are cancelled and has informed anyone booked to travel on dates up to and including Wednesday.
Ryanair said they will issue the next set of cancellations tomorrow.
There is a clause in the aviation regulations which says the airline does not have to pay compensation if there are exceptional circumstances behind the cancellation and the airline has taken all reasonable steps, but the Commission said that clause does not apply in the case of flights cancelled because of the leave issue at Ryanair.
The disruption is because of difficulties Ryanair is having managing a transition of its leave year from an April to March model to the calendar year of January to December as required by European regulations.
Ryanair has said it messed up and apologised to passengers.
The compensation starts at €250 for flights up to 1,500km, €400 for longer EU flights and €600 for flights outside the EU longer than 3,500km.
That compensation can be reduced by 50% if the airline manages to reroute the passenger and get them to the original destination within two hours for short flights, three hours for longer EU flights and four hours for those flights outside the EU longer than 3,500km.
The regulations state that the compensation paid to passengers is separate to reimbursement of passengers' expenses.
"We will cancel 40 to 50 flights daily for the next six weeks, less than 2% of our schedule, with a slightly higher number initially, as we begin to implement these cancellations," said a spokesperson for Ryanair.
For anyone booked to travel with Ryanair between tomorrow and the end of October, Ryanair said it will contact passengers directly.
It said the vast majority of its flights will operate as normal and those passengers who are affected will be offered a refund or seats on a later flight.