Passengers looking to fly from London to Dublin on Tuesday were blocked from boarding the plane after they failed provide a negative test result for Covid-19.
Ryanair has confirmed "a number" of people travelling from Stansted were refused entry to the plane, according to Dublin Live.
The episode highlights public confusion about what they can and can't do as restrictions are lifted after lockdown.
Under the law, all passengers arriving in Ireland must have a negative or 'not detected' result for Covid from an RT-PCR test within 72 hours of flying.
Those who don't will be barred from boarding the plane, as passengers discovered on Tuesday.
Have you been stopped from flying due to Covid restrictions? Email webnews@mirror.co.uk
A Ryanair spokesperson said: "Ryanair fully complies with government restrictions.
"A number of passengers on this flight from London Stansted to Dublin Airport were denied boarding as they failed to present a negative Covid-19 RT-PCR test result, as required by Irish regulation."
She added: "Ryanair urges all passengers to check the latest travel updates on the Ryanair.com website and with the relevant authorities in advance of their flight.”
The incident on Tuesday coincided with Ryanair CEO Michael O'Leary threating to pull his planes out of Ireland if the Minister for Transport, Eamon Ryan, didn't produce a plan for the return of international travel.
He told Newstalk Breakfast: "We in the tourism industry are bitterly unhappy."
He said quarantine rules that say UK visitors must isolate for two weeks when they arrive in Ireland mean restrictions on air travel were not necessary, adding that most UK adults have had their first vaccine.
In the UK the number of new confirmed infections went up by 18 per cent in a week - from 2,696 on May 20 to 3,180 on Wednesday.
Hospital admissions saw a week-on-week rise of 11 per cent.
