Thousands of Ryanair's Irish passengers face delays and knock-on disruption if the airline's Dublin-based pilots go ahead with a planned 24-hour strike.
However, only those travelling between Ireland and the United Kingdom will be hit by the stoppage.
They are taking action following a conflict over management's approach to the transferring of pilots between it's African and European bases.
The airline has agreed to meet Forsa, the trade union, at a neutral venue at Dublin Airport today or tomorrow to try to resolve the dispute before the planned 24-hour walkout on July 12.
"We apologise to our Irish customers for these regrettable disruptions which we have done our utmost to avoid".
All affected customers should have received a text or email about the status of their flight.
The airline says only some services between Ireland and the United Kingdom will be affected, while all Ireland to Europe flights will operate as normal.
The Irish pilots' union (IALPA) supports the strike, although Ryanair claims that only 27% of its own pilots are behind industrial action.
In Fiery Interview With British Tabloid, Trump Criticizes Prime Minister, London Mayor
The U.S. can not negotiate a trade deal with the United Kingdom until that date Theresa May cited, which is in 2019. May's plan from her advisers and that he now felt a trade deal with the US would be possible.
However Ryanair has said employment conditions for its staff are competitive, if not better, than those offered by rival carriers.
Customer service teams are working until 10pm to help with refunds, flight transfers or booking ferries.
Ryanair also say customers who haven't received notification of a cancellation should check in as normal on Thursday. Unions make clear that further action should be expected if Ryanair does not take onboard some of their demands.
"Ryanair pilots have already secured a 20% pay increase, earn up to €200,000 p.a., work 5 days-on, followed by 4 days-off (a double bank holiday weekend at the end of every week), enjoy rapid promotions and unmatched job security".
It said it is cancelling some flights on high frequency routes from Ireland to London and other destinations in the UK.
"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".
The company said it could not rule out further disruption this month or next.