SIPTU has claimed that up to 20 outside workers were brought in to operate the Cheestrings production line at the Kerry Group plant in Charleville as workers began their third 24-hour stoppage at the plant.
SIPTU said the move would only exacerbate an already difficult situation and prolong a dispute over the company's refusal to engage in set dispute relations procedures.
Today's stoppage was the third held by workers in a fortnight at the Co Cork plant.
Following last Monday's day-long picket, which shut the plant, the company issued protective notice to the workforce and advance notice of layoffs to take effect from next Tuesday, 14 February.
Some 200 of the 250 workers at the plant voted overwhelmingly for industrial action after the company withdrew from a hearing at the Workforce Relations Commission last November.
Both sides had been engaged in protracted discussions over planned changes in working conditions for 68 workers on the Cheestrings production line, which would have also impacted on other workers.
SIPTU organiser Terry Bryan said pickets were placed on the plant at 7am today.
The strike committee were to meet today to discuss the latest developments and make a decision on escalating industrial action.
He urged Kerry Group to reconsider its position and return to the Workplace Relations Commission.
However, the company said earlier this week that it had made every effort throughout 2016 to resolve the dispute but withdrew from a planned WRC meeting last November in frustration at a lack of progress.
A spokesman confirmed that a number of Kerry Food management employees from within the group have been drafted onto the site this morning.
He said the stoppages had disrupted the week's production and management are determined to secure the viability of the plant by managing and maintaining production standards.
