Around 320 SIPTU members at Roadstone Woods have gone on strike in a dispute over bonuses.
SIPTU official David Lane said the row revolved around bonuses worth around €2,500 a year, which were cut by 89% two years ago as part of a bid to cut payroll costs by €6m.
The union argues that as the €6m target has now been exceeded, the bonuses should be restored.
Mr Lane said that at talks last week Roadstone Woods had said it was now seeking a further €10m in payroll savings.
A spokesperson for Roadstone Woods said it would not be commenting at this point.