Ryanair is facing its first day of industrial action in Ireland since the airline bowed to pressure from staff and recognised trades unions.
The budget airline has cancelled 30 flights on Thursday after the Irish Airline Pilots' Association voted to strike last week.
Only flights between Ireland and the UK are affected.
However, Ryanair is facing further strikes in Spain, Portugal, Italy and Belgium later in the summer.
The airline has apologised for the "regrettable disruptions", saying it was trying to minimise the strike's impact on people travelling to Portugal, France, Spain, Italy and Greece.
The cancellations are all on high frequency routes between Ireland and UK destinations including London, where passengers can easily switch to alternative flights.
The Irish pilots' concerns relate to pay and conditions including a dispute over seniority, as well as the procedures for allocation of base transfers, promotions and annual leave.
Last autumn, Ryanair was forced to cancel hundreds of flights after rostering failures left the airline with insufficient crew. At the time staff made it clear they were unhappy about their working conditions.
Despite a history of hostility towards unions, the airline eventually agreed to recognise them in December.
This year the airline has suffered some minor industrial action in Germany and Portugal, but unions are gearing up for a series of strikes that could affect routes across the continent.
Cabin crew in Italy will strike for 24 hours on 25 July, while cabin crew in Spain, Portugal and Belgium will strike for 48 hours on 25-26 July.
Unions have said further action may follow if the airline does not make concessions on some employment terms.
One of their requests is that they be paid into bank accounts in their own countries, rather than in the Republic of Ireland.