Hundreds of jobs are at risk at Bord na Móna and the ESB, after the redevelopment of an Offaly power plant was refused.
Bord na Móna has said it is "reviewing" a decision over the ESB's West Offaly Power Station.
A plan to switch from turf to biomass at the plant has been rejected by An Bord Pleanála.
The board found that pollution during construction or operation has the potential to lead to a drop-off in vegetation and fish in the water.
It also concluded that discharges from supply bogs have the potential to "reduce foraging resources".
Bord na Móna said it will be providing briefings to employees "in the coming days."
It will also be consulting with a range of stakeholders as it considers "all options in relation to the impact of this development."
SIPTU energy sector organiser Willie Noone said the plant could shut down as early as next year.
"The just transition appears to be to throw the workers just off a cliff.
"I think this is the most serious development that could have happened - the timescale has now just gone out the window.
"This is going to have a serious affect on the Midlands - this'll affect both Lanesborough Power Station and Shannonbridge now because the two of them are interlinked.
"The ESB has serious decisions to make about whether that power station will even operate in 2020".
He has called on Environment Minister Richard Bruton to visit the area.
"I think he'd need to take a journey and travel down from Dublin and walk through the streets of the Midlands and face the people.
"And actually see the shock that will be on their faces when news breaks, because I don't believe that he can actually sense the devastation that would be felt at this news".
Local TD Denis Naughten has described the decision as a "devastating blow" to the whole region.
"This decision has far reaching implications for 1000's of jobs and families across the midland counties and the very first step that must now be taken is to protect the existing jobs pending a full assessment of the implications of the refusal to extend planning for the West Offaly Power Station".
"The Taoiseach must today convene an emergency meeting of Cabinet Sub Committee A on the economy to come forward with proposals to be presented to the full Cabinet later this week, to secure local jobs in the short term, while the current road map for the transition away from peat is re-evaluated".
This is the second potential blow for Bord na Móna workers in as many weeks.
Last week, workers accused the company of using the problems at its Mount Dillon operation to lay-off long-term staff members without pay.
Earlier this month, the company said it was laying off around 150 workers due to the closure of the nearby Lough Ree Power Plant.
Bord na Móna's Mountdillon works supplies peat to the power plant.
The company has already let 78 seasonal workers go and unions claim management is planning to move forward with plans to lay off 18 permanent staff.
It had originally planned to let 72 permanent staff members go - but alternative arrangements have been found for just over 50 workers.