The Office of Public Works has warned local authorities that the River Shannon is expected to reach peak levels on Sunday night and early on Monday morning.
Homes and businesses around the river in Limerick, Clare and Athlone are already under water after Storm Desmond last weekend.
Even more bad weather is forecast for the coming days in the area.
The ESB will update local authorities later on its plans for releasing water at the Parteen Weir, which could cause even more flooding.
The army, Civil Defence and the Fire Service have been deployed to help struggling communities.