One passenger has described conditions at Dublin Airport as a 'nightmare' after a number of flights were cancelled or delayed.
It follows earlier problems with air traffic control in Britain, which the operator said has now been rectified.
In an updated statement, the UK's National Air Traffic Services (NATS) said the "technical issue" affecting its flight planning system had been "identified and remedied", but travellers continued to face disruption.
Air Traffic Control system update at 15:15 UK time
We have just published an update on our website - please read it here:https://t.co/YJO7NyZKxs
— NATS (@NATS) August 28, 2023
"We are now working closely with airlines and airports to manage the flights affected as efficiently as possible," NATS said.
"Our engineers will be carefully monitoring the system's performance as we return to normal operations.
"The flight planning issue affected the system's ability to automatically process flight plans, meaning that flight plans had to be processed manually which cannot be done at the same volume, hence the requirement for traffic flow restrictions.
"Our priority is always to ensure that every flight in the UK remains safe and we are sincerely sorry for the disruption this is causing. Please contact your airline for information on how this may affect your flight."
These people are at Terminal 2 in Dublin Airport said there was very little information.
"We were at a wedding in Wicklow and we drove here this morning... there was not really any information about the flights," one man said.
"We weren't aware of any delays or cancellations."
"Our friends were on another flight that got cancelled, they got on their flight and then they got told to get off," another passenger said.
"They text us to let us know," she added.
Another man described the scene at the airport: "They told us no, we can't get anywhere.
"Now there's hundreds and hundreds of people trying to find new flights, new hotels to stay in for the night - [it's a] nightmare."
The UK national air traffic control service @NATS is working to resolve a technical issue. UK airspace is not closed but @NATS has applied air traffic flow restrictions to ensure they can maintain safety. Please contact your airline for the very latest status of your flight. https://t.co/sOJC80avBY
— Dublin Airport (@DublinAirport) August 28, 2023
Travel expert Eoghan Corry told The Hard Shoulder the situation could have been much worse.
"It was data processing, it wasn't anything that would mean everything had to be grounded straight away," he said.
"It did mean they put caps on the amount of movements, and the delays started mounting up.
"We are incredibly affected here, because 30% of Ireland's air traffic is to and from the UK and [there's] an awful lot of overflights.
"We've lost 21 departing flights out of Dublin Airport - that's 3,500 departing passengers who had their flights cancelled - including places like Munich, Amsterdam, Paris.
"Tens of thousands of passengers had a long day with very little information, delays and no real updates on what was happening."
Asked if flights could go around UK airspace, Eoghan explained: "The uncertainty and the congestion meant that trying to get the huge amount of air traffic to and from the UK, it couldn't be done.
"They actually managed to keep a lot of the traffic going - the effects could have been far worse."
Eoghan said there will be ripple effects felt for the next 24 hours or so.
"An aircraft in the wrong place for six or seven hours today has a big knock-on effect to flights that don't even go near the UK.
"EasyJet has just cancelled all departures today, so overnight aircraft will be in the wrong place.
"There'll be a lot of scratching of heads and people out with their abacus, trying to work out how to get aircraft back to where they're supposed to be."
Stuck on the tarmac in Liverpool for 45 mins so far due to system failure with air traffic control… pic.twitter.com/JRO89TRLIB
— Alex Rowley (@AlexRowley04) August 28, 2023
Newstalk journalist Alex Rowley is on a Ryanair airplane at Liverpool John Lennon Airport, and told the show it has been a long wait.
"We're just about to take off about three and a half hours after we were supposed to originally," he said.
"We boarded on time - we became aware of it in the terminal when a Cork flight got completely cancelled.
"My friends were on an Aer Lingus flight before me, they actually didn't end up boarding until about an hour and a half after me.
"They're still on the tarmac, they haven't been taxied out - we've been taxied out."
Alex said people were given updates, as well as the option to disembark.
"They had to turn on the [call bell] light if they wanted to get off, so a load of people disembarked.
"Constant updates throughout in fairness, but for a good hour and a half there it was very tense on the flight to say the least.
"They weren't allowed back on - they got their bags, they got off and back into the terminal," he added.
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