Up to 900 jobs are at risk at Ryanair.
The airline says it is because of the risks of a no-deal Brexit and the Boeing 737 Max planes being grounded.
In a video sent to workers, Ryanair's chief executive Michael O'Leary outlines plans to cut the number of staff and planes.
He says they currently have 900 pilots and cabin crew that are not needed and that 600 will need to be laid off by next summer.
This comes after Ryanair announced on Monday a fall in profits of 21%.
Michael O'Leary says one of the reasons cuts are needed is because of uncertainty over a no-deal Brexit.
The grounding of the Boeing 737 Max after two fatal crashes is also having a significant impact.
It is not yet known how many of the cuts will affect Irish workers but a final decision is expected by the end of next month.
The Irish Airline Pilots Association says it is in discussions with Ryanair to find out more.
The trade union Forsa said the threat to jobs is concerning.
"Today’s development is obviously of concern to Ryanair staff, including Fórsa members.
"At this point the union has no specific information about the routes or bases that may be affected.
"Fórsa is watching the situation closely, and has told the airline’s management that it expects to be consulted on any measures that could impact on the jobs, incomes or working conditions of union members."
The union said it would make no further comment at this point.
This article was amended at 3.40pm on July 31.