Workers at a pharmaceutical ingredients plant in north Dublin have been issued with Protective Notice.
In a statement SK Biotek said it has begun laying off workers temporarily at its plant in Swords from yesterday.
Up to 350 people are employed at the plant and 150 of those who work in the core manufacturing and maintenance areas have been issued with the notice.
SIPTU, which represents 87 workers, said members began a picket on Thursday due to what it termed as a "lockout by management".
The company said it is "disappointed" that unions have engaged in industrial action rather than continuing to take part in a consultation process.
SK Biotek said the process would be to the "long-term benefit of the business and its employees."
However SIPTU said the "dispute with the company relates to its failure to respect the terms of a collective agreement."