Up to 60 workers at a Newtownabbey-based company could be made redundant.
Workers at Ciena, based at the former Nortel plant in Monkstown, in County Antrim, were told the news on Monday.
The technology firm, which bought over part of the Nortel business in 2009, is planning to lay off between 50 and 60 workers, representing about half of the current Monkstown workforce of 118.
Representatives of the union Unite said they were told the jobs would be lost because of a re-organisation.
The jobs are expected to go over the next three to six months.
Sean Smyth from the Unite union described it as "dreadful news."
Ciena said that it is reorganising its global distribution model and confirmed that between 50 and 60 staff could lose their jobs as a result.
The company said it is currently in consultation with the workers' representatives and where possible it would be try to re-deploy people.
Last year, 17 jobs were lost at the company when they were transferred to Baltimore in the United States.