The ADM chemical plant in Cork has ceased production due to a row between management and unions over new working practices.
ADM, which employs more than 160 people in Ringaskiddy, issued protective notice to its workers but later reversed the move.
The company has drawn up a restructuring plan that involves new shift-working arrangements but SIPTU, which represents almost half of the workers, accuses management of trying to "force through" a restructuring plan.
A ballot over the new arrangements on Tuesday was rejected, and SIPTU claims some of its members have been effectively locked out of the plant since since 8 a.m. on Wednesday.
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It is believed management want the new working practices in place before any talks with the unions.