Ninety jobs are to go at Merck Sharp and Dohme in Brinny in Co Cork over the next six months.
Management briefed staff about the plans at a meeting this afternoon.
The company says the move is necessary for the site to stay competitive. Active pharmaceutical ingredients are made at the Brinny plant.
Merck has a total of seven plants in Ireland, employing more than 2,000 people.
More than 455 people are employed at the Brinny site, where the company established a site in 1980.