The Transport, Engineering and Electrical Union believes Lufthansa will shut down its aviation plant in West Dublin, after its 400 staff were warned the company is considering closure.
Staff were called into a meeting this afternoon and told the jet engine overhaul company is no longer viable, after running at a loss this year.
Negotiations are due to start on Monday, to see if a future can be secured for the plant in Rathcoole.
Arthur Hall from the TEEU believes the company has already made up its mind to shut down, and it has come as a complete shock to staff.
Mr Hall said: "It's the worst Christmas present ever, the vast majority of those people have families, so it's the worst possible timing by the company.
"There are people working here over 35 years who thought they would see out their working life here and they would get their pension, but this is just devastating news.
"The atmosphere inside on the factory floor this afternoon was very distressing."