Ciena makes equipment for phone and data networks
Almost 20 jobs are to be lost at a telecommunications firm in Monkstown.
Ciena, which bought over part of the Nortel business, says it is to shed 17 jobs. The posts are to be relocated to Baltimore in the US.
A spokesman for the Unite union said he was fearful that 120 remaining jobs at the site could be lost by the end of the year.
Unite's Sean Smyth said: "Our members are reacting angrily to this announcement and fear the worst."
He added: "They believe that President Obama's call on protectionism for US workers mean that more job loses at Ciena and more US-linked firms in Northern Ireland are now under major threat.
"They are demanding that the Stormont Assembly set up a jobs task force immediately to deal with the issue."
Ciena is a Nasdaq-listed company based in Maryland. It agreed to buy Nortel's ethernet division for $774 million last year.
Another US firm, Avaya, which bought other parts of Nortel's business said in January it was considering closing its Monkstown operation.
About 140 people work for Avaya in Monkstown.
Avaya, owned by private equity firms Silver Lake and TPG Capital, bought Nortel's enterprise division out of administration for $915m in December 2009.