Aer Lingus has apologised as problems on its website and app left passengers unable to download or print their boarding passes.
While customers were able to check in, an error message told them “all attempted boarding pass retrievals failed”.
The Irish airline said passengers will have to be issued with their pass at the airport, which left some people pointing out that such circumstances could lead to queues.
One tweeted: “the thoughts of the extra queue tomorrow if we can’t get them today. Fun.”
Another wrote: “I’d say it could be panic stations? Looking at their Tweets & Replies I’m not the only one w this problem.”
The airline’s website advises: “If you choose not to check in online or on your mobile, please allow enough time for airport queues.”
Responding to customer queries to its official Twitter account on Sunday evening, the airline said it was “currently experiencing problems issuing boarding cards on our website and App”.
Advising people to collect their boarding card from an Aer Lingus agent at the airport, they added: “Apologies for any inconvenience.”
It comes a day after British Airways had to apologise after it emerged that some passengers were wrongly told their flight would be cancelled following the announcement of pilot strikes next month.
The airline was urged to “sort out this mess” after the email error which saw people advised to rebook their flight, only to be told later that their original flight was going ahead as planned.