The secretary of the Bord na Móna group of unions says up to 1,000 jobs could be lost in the Midlands in a five to six-month period.
It follows the decision by An Bórd Pleanála to refuse planning permission to redevelop an ESB power plant in Shannonbridge, Co Offaly.
Speaking on RTÉ's Morning Ireland, Willie Noone said up to 800 direct jobs and another 200 indirect jobs will be affected by the decision.
"There's roughly around 800 people employed in the peat part of Bord na Móna. All of those jobs are now on the line," said Mr Noone.
"And if you take into account the indirect jobs - contractors etc - you could be talking about 1,000 jobs being lost in the midlands in a five to six-month period."
Mr Noone says he hopes to meet management of Bord na Móna on Friday and said he hopes the decision to refuse planning permission will be appealed.
The power station in Shannonbridge had looked for permission to continue to burn peat past a December 2020 deadline until 2027, while it moved over to burning biomass fuel instead.
Yesterday, Minister for Communications, Climate Action and the Environment Richard Bruton said he was disappointed that permission to redevelop the power station has been refused.
Mr Bruton said it is still a priority of the Government to support Bord na Móna's plan to have an "orderly exit" from peat and to provide alternative employment.
"The ESB will have to look at this and see what the implications for the future are. I think it's premature to be drawing any conclusions," he said.
Bord na Móna said it and the ESB will now study the implications of this decision carefully.
In a statement Bord na Móna said: "Ireland's recent acceptance into the EU Coal Regions in Transition Platform, will assist with technical expertise and will provide for the exchange of knowledge and ideas as we develop a pathway forward.
"The minister and the Government's overall objective is to ensure Bord Na Móna implement its Strategic Plan for the planned phased transition away from peat and the creation of new, long-term, sustainable employment opportunities for the midlands into the future."