Hundreds of Aer Lingus passengers waited in long queues and flights were delayed at Dublin airport yesterday after the check-in computer crashed for nearly four hours.
Aer Lingus staff were forced to revert to manual check-ins.
The airline said over 1,500 passengers were affected by the systems failure, with 25 flights delayed with knock-on effects.
The system went down at Dublin and Shannon airports at 10.45am and resumed at 2.25pm. An Aer Lingus spokeswoman said an investigation had begun into the breakdown.
At Shannon, transatlantic flights suffered only short delays. However, in Dublin 25 flights to UK and European destinations were delayed for 1½ hours, with a knock-on effect on returning and outgoing flights.
Five flights were held to wait for passengers. They were flights to Paris, Zurich and Amsterdam and two to London Heathrow.
Many queues to check-in desks formed and wound around the departure terminal. Dublin airport police at the central doors stopped people entering because of congestion and directed them to another entrance.
Aer Lingus and Dublin Airport Authority customer-service staff were in the terminal but at times passengers could not reach them through the crowd.
Passengers complained they were not given information about flights. Gaston Serpenti waited from noon for his flight to Amsterdam, and he was still in the queue at 1.50pm for his 2.10pm flight.
"When I arrived we were told we should wait downstairs because the check-in computer had crashed and told we would get information. We waited for 1½ hours. Then a stewardess came from Aer Lingus and told us to start checking in. So it's two hours now."
Jessica Goldrick, from Dublin, was in the queue for her flight to Budapest. She started queuing at 1.20pm, and was a long way from the check-in 40 minutes later. "My flight is at 2.50pm and I'm in meltdown. It's the worse thing I've ever seen."
Carmel Verdon and Pauline Kavanagh from Dublin were on their way to Alicante. Ms Kavanagh said: "We are in the dark. There was a loudspeaker, but there was so much noise we couldn't hear anything. A lot of people were skipping the queues and we didn't know whether we were in the right one. These things happen, but you would think there would be a system in place for dealing with things like this."
An Aer Lingus spokeswoman said passengers were kept informed. "Announcements were made every 15 minutes over the public address system telling them that flights would be accommodated."