The site has been under Care and Maintenance since last Summer.
Some workers at Tara Mines could be facing voluntary redundancies as the operator attempts to reopen.
The site was placed under care and maintenance last summer with 650 staff temporarily laid off.
SIPTU claims the company didn't sit down with unions before going public with the news.
Sector Organiser John Regan says it's a slap in the face to staff.
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