Bord na Móna has announced that up to 430 workers are to be made redundant, with up to 150 expected to go by Easter next year.
The job losses follow a decision by the company to close 17 of its 62 active bogs immediately, with harvesting at the remaining 45 bogs expected to stop by 2025.
Bord na Móna Chief Executive Tom Donnellan said the decision has been made necessary because of the need to decarbonise energy sources in response to climate change.
The chairman of the Bord na Móna group of unions said he is shocked by the scale of job losses proposed by the company.
Willie Noone of SIPTU was speaking at the Boora Bord na Móna works in Co Offaly this afternoon after formal discussions on the job losses began with company management.
Mr Noone said unions fear that the number of job losses in the short to medium term will not stop at 400.
He said he believes up to 850 workers who are working in the peat sector will lose their positions and has rejected suggestions by Bord na Móna that up to 400 of them could be redeployed.
He said a significant proportion of the workers in peat energy are aged in their 50s and beyond and added: "We do not believe that a significant majority of these workers can be redeployed".
Mr Noone said: "We are calling on the company to come forward and provide a proper voluntary severance package which will reflect the quality of the contributions made by these employees over the last 30 years and more."
Mr Noone said he was satisfied that the company is prepared to engage in a process of negotiation with them, but there were clear question marks over Bord na Móna's proposed move into the biomass industry that would have to be addressed if any workers were going to have a secure future in the company.