Pharmaceutical giant Schering-Plough has axed more than 160 jobs at its west Cork plant.
Management at the facility said it was seeking the redundancies over the next three years as part of a cost reduction programme.
The news was broken to workers during a mass meeting at the plant at Brinny, near Innishannon.
Schering-Plough, which employs 519 people in the city, has been manufacturing in west Cork since 1972.
Management said the company, which last year merged with Merck & Co, will attempt to cut its workforce through voluntary redundancies.
John Howell, site director, said while the transformation programme will be extremely challenging, it is an opportunity to secure further investment in the future.
"We are committed to treating affected employees with respect and providing them with appropriate resources during the transition period," he added.
The Cork plant manufactures cancer care and anti-viral drugs and at one stage employed more than 1,000 people.
Former employee, Labour Party Senator Michael McCarthy, said the job losses are a massive blow to workers and the local economy.
"While there is often a sense of resignation when we lose low-skilled jobs in the traditional manufacturing sector, these jobs are well-paid positions, held by skilled and qualified workers, and are high in employment value chain," he said. "When we start haemorrhaging jobs like this, in an international export-driven sector, we really do have to sit up and take notice."