Severe floods are expected to hit thousands of homes and businesses along the Shannon right through to the weekend.
Emergency response chiefs issued the warning as more rain fell on saturated lands in the catchment area from Leitrim to Limerick with the worst affected region likely to be from Athlone and further south along the river.
The Office of Public Works classed the impact as a worse than 20-year event.
The river from Lough Derg to Limerick city is expected to peak on Sunday and Monday and while flow of water from the Parteen weir is not expected to be increased from its current huge volumes, people living downstream are already bracing themselves for floods.
Some of the worst hit areas include Athlone, where strong winds were compounding the problem of a rising river and scores of homes and businesses in the oldest parts of the town were left to defend themselves with sandbags but expect the worst.
Up to 35mm of rain – nearly a third of what is normally recorded in December – is expected to fall on parts of Connacht, Donegal, Clare and Kerry into tomorrow, deepening the crisis for hundreds of homes.
In more rural areas thousands of acres of farmland was already under water with many people resigned to the fact of another flood in six years.
Councils in Clare, Limerick and Ballinasloe called in the Army to help with defences.
In Limerick seven pumps were in operation shifting water away from the villages of Castleconnell and Montpelier while an inflatable flood boom, which acts like movable defence wall, was being used to channel some of the floods.
Pictures from the Irish Air Corps taken over the Shannon catchment area showed the extent of the steady build-up of flood waters in the wake of Storm Desmond.
Iarnrod Eireann said the Sligo line from Carrick-on-Shannon to Longford would remain closed until the weekend at the earliest due to flooding with the water about a foot over the track.
In Co Galway dozens of stretches of road have been closed.
The National Co-ordination Group, responsible for handling the national response to the flooding and effects of the adverse weather conditions, are meeting daily.
The ESB warned the amount of water it was releasing over Parteen would likely cause flooding on roads, land and property close to the banks of the Shannon in Springfield, Montpelier, Castleconnell, Mountshannon, Lisnagry, Annacotty and the University of Limerick.
Limerick City and County Council warned it expects the waters to keep rising over the next few days.
“Council staff, assisted by members of the Defence Forces, are continuing to implement flood defence measures along the river bank and coordinate with local property owners,” it said.
The council also advised the public, particularly children, to avoid all flooded areas and to abide by the instructions provided by the emergency services and council staff.
In total, while levels of some tributaries are showing signs of falling, the waters along the Shannon are expected to rise half a metre in some parts by late tomorrow or early on Friday.
The Defence Forces said troops had been deployed to support flood response work in four counties and remained on stand-by to help in other areas.
“Defence Forces mobility, manpower and specialist engineer assets remain available to assist our communities if required by local authorities,” a spokesman said.
In Clonlara, Co Clare, 15 troops were supporting local efforts, while 30 had been dispatched to Castleconnell, another 12 in Ballinasloe, Co Galway, and 35 in Athlone, with others put on notice in Donegal, Galway, Limerick and Cork.
Outside of the main Shannon basin, towns and villages along the Lee in Cork were also braced for floods while large swathes of rural Mayo were also under water.
Homes braced for more Shannon floods through to the weekend
