The ESB has announced that it is to close the power generating stations at Great Island in Wexford, Poolbeg in Dublin and Tarbert in Kerry by 2010.
The move is part of an agreement with the energy regulator to reduce the ESB's dominant position in the marketplace.
Nearly 360 jobs are to go if all the closures annouced today are carried out.
The steam turbine at Marina in Cork is also to close but the larger gas turbine there will remain open. These plants currently generate 1300 megawatts of electricity.
In exchange for closing the stations, the ESB will be allowed to construct a new station in Aghada, Cork Harbour with a capacity of 430 megawatts.
This will bring the ESB's power generation share of the energy market to 27% by the year 2010 down from its current level of 44%
Unions at ESB will meet next Thursday to consider their views on the closures, on which they were briefed today, and which they say they will consider in detail in the context of the government White Paper on Energy.
The White Paper includes plans to transfer formal onwership of the national grid to Eirgrid, a move which is strongly opposed by unions.
Davy Naughton, Secretary of the ICTU / ESB group of unions, said today that the unions will not allow this to go ahead, saying it amounts to "asset stripping".
However, Minister for Energy Eamonn Ryan said today that the transfer must go ahead. Minister Ryan stressed that the national grid will remain state-owned.
