A power station in Co Longford is free to reopen immediately, according to a union representative.
Lough Ree Power Plant in Lanesborough closed last month because of environmental concerns.
Nearly 150 jobs are in jeopardy at a nearby Bord na Móna operation which supplies the facility.
Yesterday, the Environmental Protection Agency (EPA) gave the ESB guidelines on a revised licence, which could enable the plant to reopen in early September.
But SIPTU's Willie Noone believes it should reopen immediately, because the EPA never forced it to close.
"My expectations are that the ESB would expedite getting the information out to the public regarding when they intend to have Lough Ree Power Station operating again," said Mr Noone.
"There is no reason why they have to wait until the end of the consultation period which is on September 3.
"There is no impediment now to operating that station immediately."