Nearly 700 workers at Tara Mines have been put on protective notice by management.
However, work at the Navan zinc mine is continuing and is being reviewed on a daily basis.
The owners, Boliden Tara, are seeking cuts in pay and conditions for the 670 staff who work in the Navan-based operation.
The matter has been referred to the Labour Relations Commission and it is understood that talks are due to take place next week.
However, the company has issued protective notice to all their employees.
SIPTU, which represents about 400 of the workers, has criticised management's decision.
John Regan said the move to put everyone on protective notice was "totally unacceptable" and flew in the face of normal industrial relations.
He said that an agreement had been signed between the company and its staff seven months ago, which was to last to 2014 but now management were seeking to "tear it up and start again".
So far there has been no comment from the company.