Around 160 jobs are expected to be lost at Tara Mines as part of an agreement to reopen the facility after nine months in care and maintenance.
The redundancies would be on a voluntary basis.
Around 50 workers are thought to have retired or taken early retirement since the plant was temporarily closed and placed into care and maintenance nine months ago.
An agreement to reopen the mine was reached overnight following talks between unions and management at the Workplace Relations Commission.
As part of the deal, core pay for remaining staff will be maintained but there are expected to be changes to work practices.
A letter circulated by Boliden Tara Mines to employees in the early hours of this morning confirmed the agreement on a set of proposals that, if accepted, would enable the mine to reopen.
They were informed that it would take "some time" for the operation to resume in full, but it was the company's intention to commence the reopening process "immediately on acceptance of the deal".
Employees were told that under the new agreement, the terms of a proposed voluntary severance scheme would be "enhanced" for those who wish to avail of it.
The letter stated the company believes the proposals were the "best possible in light of the continuing external market challenges" and would allow the reopening on a financially sustainable basis.
SIPTU divisional organiser for Transport and Energy Adrian Kane said the proposals contain "significant changes in work practices".
Speaking to RTÉ's News at One, he said the core terms and conditions of employment had been secured but work practices "will be changing", adding that the matter had to be discussed with unions members.
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He said "most people" had found alternative employment during the months of negotiations.
"That particular issue is dealt with in the proposals as well in terms of how available people are and commitments to other employment also," he said.
Mr Kane said there would "unfortunately" be redundancies but added that they would be on a voluntary basis.
"Some people would have gone on early retirement and just normal retirement as well over the number of months," he said.
"We would envision that the level of redundancies would be somewhere near the 160 mark."
Mr Kane said the mine would likely take "somewhere in the region of six to seven months from when people will start going back" to fully reopen.
SIPTU said the details of the proposals will be finalised in the coming days.
'Significant improvement'
The Unite trade union, which represents craft and staff grades at Tara Mines, said the proposed agreement represents a significant improvement on the scheme proposed by Boliden in January.
"If the agreement is accepted by workers, we would urge Boliden to set a firm date for the re-opening to be completed, and to give workers an assurance that the company is committed to Tara Mines' long-term future," Unite regional officer Brian Hewitt said.
Representatives of the Tara Mines Group of Unions are now beginning a consultation process with their members on the deal.
Shop stewards and section representatives have been called to attend a meeting on Tuesday 23 April in Navan, Co Meath.
This will be followed by a general meeting of the members on Thursday 25 April.
A ballot on the proposals will commence on 25 April and conclude with a count on Friday 3 May.
The Tara Mines Group of Unions includes SIPTU, Connect and Unite trade unions.
Tara Mines closed temporarily in mid-July 2023, resulting in the laying off of 650 workers.
The owners, Boliden, said it took the decision due to unsustainable losses and cited factors including a decline in the price of zinc, inflation and energy costs.
It said that the temporary closure would safeguard the future of the mine.
Additional reporting Laura Hogan
