Ryanair is cancelling up to 30 of its 290 Irish flights on Thursday after its pilots voted for strike action.
The budget airline said only some UK-bound flights from Ireland would have to be grounded, after 94 of about 350 pilots voted for strike action.
Ryanair said all Ireland to Europe flights will operate as normal.
Ryanair said on its website: "We have tried to avert this disruption, which is unnecessary given Ryanair pilots’ and their union Forsa has received written proposals on seniority, annual leave and base transfers, which are what Forsa claims are the reasons for this strike, yet Forsa has rejected 21 separate invitations to meet Ryanair to negotiate these documents."
The airline and the union have agreed to meet on Wednesday at Dublin airport to try to negotiate out of the strike. However, the union has said it thinks it is likely the strikes will go ahead.
Ryanair said it would minimise the disruption by cancelling a number of flights on high frequency routes from Ireland to London and other UK destinations where customers are able to transfer easily onto other flights.
All affected customers should have received a text or email about the status of their flight.
Further disruption in July and August remains on the cards after Aer Lingus pilots also said they were contemplating one or two-day strike action during the summer months.
"It is unacceptable that competitor airline pilots are actively organising strikes by Ryanair’s pilots when these airlines will be the direct beneficiaries of any such disruption," Ryanair said.
Meanwhile, Ryanair said Air Traffic Control (ATC) staff shortages in France and Germany caused delays to 18 per cent of its flights this morning, causing "unjustified delays".
However, UK-based air traffic control company NATS hit back at the claim, tweeting:
There are no staffing delays anywhere in the UK this morning 🤷♂️🛫 https://t.co/197TpdiDLQ— NATS (@NATS) July 10, 2018