Up to 5,000 Ryanair passengers will be hit by flight cancellations because of a 24-hour strike by pilots tomorrow.
The carrier is grounding 30 flights out of a total of 290 that are due in and out of Ireland owing to the industrial action.
However, only those travelling between Ireland and the UK will be hit by the stoppage.
All flights between Ireland and European airports will take off as normal.
The strike, which begins at 1am tomorrow, will be the first by Irish-based pilots in the budget airline's history.
They are demanding that Ryanair draws up a 'master seniority list', which they claim is the norm at other airlines.
This would mean that pilots with the longest service would be given first call on holidays, promotions and transfers to other bases.
In an update to customers, Ryanair said it could not rule out further disruption.
The Irish Airline Pilots Association has advised its members to get financial advice "due to the likelihood of industrial action continuing over a prolonged period".
Ryanair, which has Michael O'Leary as chief executive, apologised for the "regrettable disruptions" and said it had done its utmost to avoid them.
A negotiating team from the airline is meeting pilot union representatives for talks today, but the union said the strike was still likely to go ahead.
"We regrettably must plan for some disruptions on Thursday, and try to minimise their impact, especially upon Irish customers and their families travelling on holidays to Portugal, France, Spain, Italy and Greece," Ryanair said in a statement.
The airline is cancelling some flights on high-frequency routes from Ireland to London, and other UK destinations including Glasgow, Manchester and Newcastle.
Ryanair said customers on these routes could transfer "readily" to other flights.
It said all customers on cancelled flights received text and email notification yesterday.
Customer service teams are working until 10pm to help with refunds, flight transfers or booking ferries.
Those who did not receive a notification but are flying tomorrow should check in as normal.
The airline said it sent proposals to Forsa but the union said they were not the basis of a settlement.
"Ryanair pilots have already secured a 20pc pay increase, earn up to €200,000 per year, work five days on followed by four days off, enjoy rapid promotions and unmatched job security," said Ryanair.
The airline blamed some Aer Lingus pilots whose airline would benefit from the disruption for organising the strike.
Ryanair's chief marketing officer Kenny Jacobs said the vast majority of flights would operate as scheduled.
"No family going on a sunshine holiday is going to be impacted," he said, adding that the numbers affected represented 10pc of Irish "traffic" and 1pc across Europe.
Forsa said strike action was a "last resort".
"Pilots they feel they have been forced into this action by a company that is either unwilling or simply unable to negotiate a fair and transparent deal like any other decent employer," the union said in a statement.