SIPTU has accused the management of Roadstone Woods of "bullyboy tactics" after allegedly threatening to close some of its operations and place all employees on protective notice.
Around 350 members of SIPTU and the TEEU at the building materials firm are in the second week of a strike over bonuses.
SIPTU Organiser Davy Lane claimed the tactics were designed to strike fear into workers.
Management at Roadstone Woods declined to comment, but it is understood that they completely reject the union claims.
Employees are seeking reinstatement of bonuses which were cut by 89% two years ago in a bid to save €6 million at the company.
SIPTU argues that as the €6m target has been exceeded, the bonuses should be restored with immediate effect.
However, the company wants to phase restoration over 5 years.
The full annual bonuses range from €3,686 for salaried staff to €5,832 for craft workers, and €4,731 for general operatives.
Mr Lane claimed that the directors of Cement Roadstone Holdings had received massive bonuses on reaching targets that had inflicted financial hardship on ordinary workers in Roadstone Woods Ltd.
He said shop stewards were grateful to hauliers who had refused to pass union pickets, and said they were prepared to attend any talks to resolve the row.