Workers at Tara Mines say temporary layoffs announced last night came as "a massive shock".
650 of them at the Meath-based site will temporarily be out of work, with some finding out through the news.
The company says the move is necessary due to "significant and unsustainable financial losses".
Michael Fitzgerald, a miner and shop steward at Tara Mines, says news was communicated poorly to staff.
"It came as a massive shock. We were very disappointed with the communication from the company - many workers heard it on the 9 o'clock news last night.
"We'll be exploring as a union all alternatives to going into care and maintenance before all production will cease.
"It will be just keeping the pumps going, with a minimal crew, just to keep the place ready until we can come back."
