Twenty-one jobs have been lost after drinks giant Diageo announced plans to shut a factory making the flavour for its stout.
The Waterford brewing operation is to shut down by the end of the year with operations transferred to St James's Gate in Dublin.
The brewery made the Guinness flavour essence for the wider brewing operations.
Sinn Fein Senator David Cullinane said it was a puzzling move after millions spent on the facility over the last 10 years.
"We were aware of the possibility of a scaling back of operations. However it was not anticipated that good quality, high tech jobs, such as producing the concentrate for Guinness, would be lost," he said.