Workers at Tara Mines in Co Meath are 'shocked and disappointed' that the company is to temporarily lay-off 650 employees.
Its owner, Boliden Group, said this was due to a combination of factors - including rising energy bills and a drop in the price of zinc.
"The Tara zinc mine in Ireland is a high-cost mine with approximately 650 employees," it said in a statement on Tuesday.
"Boliden is working actively to extend the life of the mine in parallel with ensuring its competitiveness.
"The business is currently cash flow negative, due to a combination of factors including operational challenges, a decline in the price of zinc, high energy prices and general cost inflation.
"Tara will therefore be placed under care and maintenance until further notice.
"The decision means that production at the mine ceases and that the workforce is temporarily laid off until conditions of the operation improve," the statement added.
Employees were told about the decision at a meeting on Tuesday night.
SIPTU Divisional Organiser Adrian Kane said workers had no advance warning.
"SIPTU members at Tara Mines are deeply disappointed at the news that 650 workers are to be laid off within the next few weeks and the manner by which it was communicated to them this evening," he said.
"Our members are also concerned that they were not informed in advance of this announcement of temporary lay-offs, without pay, by the company.
"Tara Mines is a critical part of both the local and national economy and we will be engaging with management to discuss how the effects of these lay-offs can be mitigated," he added.
Enterprise Minister Simon Coveney has said supports are being put in place immediately.
"We have been speaking with Tara Mines in recent weeks and I was due to meet with senior Irish managers next week," he said.
"[This] decision of the board in Sweden is a shock and my thoughts are with all of the workers affected.
"The company has tonight told me the board is convinced this is a temporary closure.
"My department will immediately send in State agencies to assist workers, especially apprentices who are in the middle of their training.
"Enterprise Ireland has told me it can have a team there on Wednesday," he added.
Social Protection Minister Heather Humphreys said her department is "on standby to help in whatever way we can."
"A dedicated team is being put in place and will be out on the ground with the company to provide information on income supports, redundancy entitlements and employment supports for their impacted employees," she said.
Tara is Europe's largest zinc mine, and the eighth-largest in the world.
More than 85 million tonnes of ore have been extracted there since mining began in 1977.
Stockholm-based Boliden Group acquired the mine back in 2004.