Britain's National Air Traffic Service (NATS) was hit by a technical problem for several hours, causing widespread disruption to flights using UK airspace that airlines and airports said would continue for some time despite the issue being fixed.
NATS had earlier had to restrict the flow of aircraft after the issue affected its system's ability to automatically process flight plans, with airlines and airports warning of delays and cancellations.
Thousands of passengers across Europe have been impacted and dozens of flights into and out of Irish airports were either cancelled or delayed.
Around 115 inbound and outbound flights at Dublin Airport were cancelled, according to airport operator daa, as passengers tried to rebook flights and make alternative travel plans.
Britain's National Air Traffic Service said the problem had been "identified and remedied" but the impact is expected to be felt into Tuesday.
The daa advised all passengers due to travel to "check the status of their flight with their airline in advance of travelling".
"Passengers travelling on Tuesday can expect Dublin Airport to be busy, particularly in the early morning period, but additional airport resources are being deployed to help them on their way and to ensure high standards for passengers travelling through," daa said in a statement.
"For any of our car park customers delayed coming back into Dublin Airport as a result of the ATC issues in the UK, we are waiving any additional parking fees they may have incurred."
Shannon Airport said the forced restriction of aircraft flow resulted in the cancellation of the outbound (FR1182) and inbound (FR1183) Ryanair Gatwick service.
A spokesperson for Shannon Airport advised all passengers due to travel from Shannon to check the status of their flight with their airline in advance of travelling.
Ireland West Airport in Co Mayo advised passengers to expect delays on some flights.
In a statement, it said: "As a result of air traffic control issues in the UK, passengers are advised to expect delays and disruption to some flights.
"Passengers due to travel today are advised to check the status of their flight with their respective airline in advance of travelling."
In a post on social media site X, formerly Twitter, Cork Airport also said some flights had been affected.
Air Traffic Control issues in the UK today are resulting in delays to some flights into and out of @CorkAirport.
— Cork Airport (@CorkAirport) August 28, 2023
We advise all passengers due to travel today to check the status of their flight with their airline in advance of travelling. pic.twitter.com/spa4qvviCa
Among the flights due into Cork but so far cancelled were Air France flights from Paris, Ryanair flights from Liverpool, London Stansted and Manchester, Lufthansa flights from Frankfurt and Aer Lingus from Heathrow.
Departures cancelled include Air France to Paris, Aer Lingus to Heathrow, Ryanair to Liverpool, Stansted and Manchester, KLM to Amsterdam, and Lufthansa to Frankfurt.
Passengers travelling through Belfast's two main airports were warned of potential difficulties and delays.
A number of EasyJet flights from Belfast International to Great Britain and European destinations were cancelled.
Aer Lingus asked all passengers to check the status of their flight before coming to the airport as delays and cancellations are likely.
They said that passengers with cancelled flights will be contacted directly by Aer Lingus.
Ryanair said: "Due to another UK ATC failure, Ryanair will be forced to delay/cancel a number of flights to/from the UK today, Aug 28.
"All affected passengers will be notified of their options to change flights (free of charge) to another Ryanair flight or receive a full refund.
"We sincerely apologise for this UK ATC failure which is beyond Ryanair's control and is affecting all airlines operating to/from the UK."
Scottish airline Loganair earlier warned its customers on X that flights may be subject to delays due to the outage.
In a statement, the Glasgow-based airline said: "There has been a network-wide failure of UK air traffic control computer systems this morning.
"Although we are hopeful of being able to operate most intra-Scotland flights on the basis of local co-ordination and with a minimum of disruption, north-south and international flights maybe subject to delays.
"If you are flying with us today, please check our website for the latest information about your flight before setting off for the airport."
Luton Airport said it was working with the authorities to "understand the impact and the timescale for normal operations to resume", while British Airways also said it was working closely with NATS to understand the impact.
Several passengers took to social media to say they were stuck on planes on the tarmac waiting to take off on what is a busy travel day due to the British bank holiday.
One passenger being held on the tarmac at Budapest said their pilot told passengers that a massive computer failure had closed all UK airspace and they faced an 8-12 hour delay.
Athletes and several high-performance staff members were kept on the runway for three and a half hours and had to disembark after being told their flight to Dublin had been cancelled.
Additional reporting Reuters
