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 airline said passengers would have to be issued with their pass at the airport, which left some people pointing out that such circumstances could lead to queues.
One passenger 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 with this problem."
The airline said that it had assigned additional resources to check-in areas this morning to minimise the disruption to passengers.
It said: "The vast majority of Aer Lingus guests are currently not experiencing any issues with the Aer Lingus website, app and kiosks.
"A technical issue which emerged yesterday evening led to a number of guests being unable to access their boarding pass on our website, app and kiosks. The issue has been minimised and we continue to monitor the situation.
"We have deployed additional resources at our check-in area and would like to reassure those guests that they can pick up their boarding passes at the airport and the technical issue will not disrupt their travel plans."
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.