Ryanair slashed its annual traffic forecast by around 5 million passengers on Thursday (January 7).
Europe's largest low-cost carrier said fresh lockdowns in Britain and Ireland would leave the countries with "few, if any" flights.
It also criticised public health measures, saying Irish travel curbs were "inexplicable and ineffective".
In a statement , Ryanair also described the new measures as "draconian".
And called on Ireland and Britain to accelerate the pace of vaccine rollouts.
Both governments have said the spread of a new virus variant forced strict curbs on travel, and say they are distributing vaccines as fast as they receive them.
The British government on Wednesday (Jan 6) introduced legislation that would enable its current lockdown to remain in place until the end of March.
Although Prime Minister Boris Johnson said he did not expect the full restrictions to continue until then.
The Irish government said people should remain home except for essential journeys until at least the end of January.
Ryanair said air traffic in January would fall to under 1.25 million passengers, with the new restrictions potentially cutting February and March traffic to as little as 500,000 passengers each month.