Ryanair is making big cuts to its flight schedule from 21 January in response to the latest Covid lockdowns.
It warned that few, if any, flights would operate to or from Ireland or the UK from the end of January until "draconian" restrictions were removed.
Customers hit by the cancellations will be advised by email of entitlements to free moves or refunds, it said.
Ryanair also cut its full year traffic forecast from currently "below 35 million" to 26-30 million passengers.
The airline said that new Covid restrictions could reduce traffic in February and March to as little as 500,000 passengers each month. It expects January traffic to fall below 1.25 million.
It said it did not expect these latest flight cuts and further traffic reductions to materially affect its net loss for the year to 31 March 2021, since many of the flights would have been loss-making.
Ryanair hit out at Irish and UK governments for the latest lockdowns.
"The WHO have previously confirmed that governments should do everything possible to avoid brutal lockdowns, because lockdowns 'do not get rid of the virus'," Ryanair said in a statement.
"Ireland's Covid-19 travel restrictions are already the most stringent in Europe, and so these new flight restrictions are inexplicable and ineffective when Ireland continues to operate an open border between the Republic and the North of Ireland."
It called on the Irish Government to accelerate the rollout of vaccines.
"The fact that the Danish Government, with a similar five million population, has already vaccinated 10 times more citizens than Ireland shows that emergency action is needed to speed Covid vaccinations in Ireland."
'Fighting for survival'
Rival low-cost carrier Norwegian said its traffic figures had been hit heavily by the pandemic, with customer numbers down 94% compared to the same period the previous year.
In December, 129,664 customers flew with Norwegian, with the capacity and total passenger traffic both down by 98%.
"2020 has been a very challenging year and we now find ourselves fighting for survival," said Jacob Schram, chief executive of Norwegian.
"The vaccination is now being rolled out across the world and is good news for both the aviation industry and those who want to travel."