Only flights between the UK and Ireland will be affected
Ryanair says it is cancelling up to 30 Irish flights on Thursday as a result of a strike by Dublin-based pilots.
A number of flights between the UK and Ireland will be affected.
However, the airline says all flights between Ireland and continental Europe will operate as normal.
The cancellations represent just over 10% of the company's 290 Irish flights scheduled for Thursday.
The strike is being organised by members of the Irish Air Line Pilots' Association (IALPA), which is a branch of the Fórsa union.
In a statement, Ryanair said: "We have tried to avert this disruption, which is unnecessary given Ryanair pilots’ and their union Fórsa has received written proposals on seniority, annual leave and base transfers, which are what Fórsa claims are the reasons for this strike.
"All customers on these flights have received text and email notification of these cancellations earlier today and our Customer Service teams are assisting them with refunds, free transfers to alternative flights on Thursday, or Wednesday, Friday and Saturday.
"For customers travelling to the UK we will also be assisting them with alternative transport on comparable operators (both flights & ferries) where there is some limited space available."
The airline adds that it apologises for the "regrettable disruptions which we have done our utmost to avoid".
Ryanair and representatives from Fórsa are due to meet tomorrow at a 'neutral venue'.
However, the union has already warned the strike on Thursday is likely to proceed.