Siptu has described as "devastating" the loss of 85 jobs at a meat cannery in Rooskey, Co Roscommon.
The Glanbia plant workers were told just after lunchtime yesterday that they would not have jobs after Christmas.
Siptu sectoral organiser Séamus McNamee said the decision to close the plant would have "dire economic consequences for Rooskey".
Glanbia chief executive John Madden said it was with regret that the company was forced to come to its decision.
"The decision is regrettable but inevitable given the difficulties in the canned meats sector in recent years. The overall market is shrinking as a result of changing consumer preferences and people just aren't eating canned meat in the amounts they have in the past. Glanbia sustained the business, even in a loss-making situation, and did all in its power to secure the business for the long term."
Mr Madden said they tried to sell the business but there was no strong interest in it.
Siptu's Mr McNamee said Rooskey had never recovered from the loss of about 400 jobs in Glanbia's pigmeat processing plant, following a massive fire there in May 2002.
"Workers at the plant have little or no future to look forward to now."
Mr Madden said there were no immediate prospects for the workers of being redeployed in another of the company's plants and that negotiations were now under way with Siptu on redundancy terms.
"Glanbia's strategy now is to focus investments on areas of activity where we have strong market positions and which have growth potential.Therefore the decision has been taken to close the canning facility at Rooskey, to review alternative options for this site and to focus our meat activities on the primary processing of pork."
