A strike by air traffic controllers in France is disrupting some flights into and out of Ireland.
The 36-hour industrial action, which began yesterday evening, will continue until tomorrow morning.
The French Civil Aviation Authority has said flight delays and last-minute cancellations cannot be ruled out.
Intending passengers are being asked to check their airline websites for updates.
Ryanair CEO Michael O'Leary said the airline has had to cancel around 400 flights across Europe and the UK today as a result of the French strike.
In a video posted on Twitter this morning, Mr O'Leary said: "The vast majority of these are overflights, they're not going to France."
He said they are flights going from "Britain to Spain, from Portugal to Germany, from Italy to Ireland".
"We respect the right of French ATC to strike, but if they go on strike, it should be French domestic flights or local flights in France that get cancelled."
They have alternatives, including the TGV, he said.
Mr O'Leary said people flying across France are having their flights "unnecessarily cancelled because the European Commission, lead by Ursula von der Leyen, will not take action".
He said overflights should be protected like they are in Spain, Italy and Greece.
Aer Lingus has confirmed that it has not cancelled any flights as a result of the French ATC strike.
Last week, Ryanair handed in a petition with over a million signatures to the European Commission, demanding that it take action to protect overflights across the union during air traffic control strikes to keep skies open.
Cancelled flights a 'complete nightmare' for Meath family
Spanish holiday plans for a group of 14 people from Co Meath were thrown into doubt after their flights were cancelled due to the strike in France.
Julie Condron from Duleek, along with her two children, were part of a group of 14 adults and children who were shocked and frustrated to receive an email from their airline, telling them their flights had been cancelled.
Ms Condron was due to depart from Dublin Airport today for a one-week holiday in Salou with her friends and their children.
She was informed by email on Sunday that the flight was cancelled due to the French strike.
After a frantic search for other flights departing from Dublin Airport today, Ms Condron and her friend, along with their three children, were left with no other choice but to book flights from Belfast to Barcelona.
"Half of our group departed yesterday (Monday) from Dublin and the flights booked out after that so the remainder of the group booked to fly out today," she said.
Ms Condron has described the whole experience as a "complete nightmare". She said it was due to "the extra cost for the hotel and taxi [in Belfast and from Barcelona to Salou], [and] the extra travel time on top of the stress of rebooking a holiday the day before you go."
They reached their destination this afternoon and have been reunited with the rest of the group, but it is unclear at this point if they will be refunded for their original flights as they booked through a third party.
Additional reporting: Éilís Sheehy, Dyane Connor
