Cement Roadstone Holdings is to seek up to 55 redundancies at its Irish Cement plant in Limerick as part of a restructuring of the company.
Employees were notified on Friday that the company would be seeking the redundancies, but were told the plant would not be closed.
The company blamed the restructuring on a massive downturn in the construction sector, where activity has dropped 80% from its 2007 peak.
It said there is little visible indication of any market improvement in the foreseeable future.
SIPTU organiser David Lane described the announcement as "shocking" and said the union would fight to ensure that members received the maximum entitlements.
Details of the restructuring will be discussed at talks with unions over the coming days.
The Irish Cement plant in Limerick has been in existence since the 1930s.
Earlier this year, over 100 workers at the Castlemungret and Platin Irish Cement plants held a six-week strike over an unpaid Christmas bonus worth an average of €5,500 per worker.
The dispute was resolved in May.