Waters along the Shannon are expected to exceed severe flood levels with up to 80mm of rain forecast to hit already devastated counties this weekend.
Families have left their homes along the old river system in parts of Clare below the Parteen weir, with Clonlara and Springfield surrounded by water and at least one household being evacuated to a hotel.
Further upstream in Athlone, where the highest water level increases were recorded in the last 24 hours, 90 homes are either being flooded or at immediate risk in The Strand and Deerpark areas.
Some of the worst side effects of the Shannon bursting its banks in the town saw raw sewage pour out into gardens and streets.
Householders in many flooded areas reported unusable tap water and toilets and heating systems breaking down.
About 10,000 homes in the Ballinasloe area are under a boil water notice while Irish Water said 84 of its treatment plants have been impacted by the huge volume and worsening quality of water due to be treated.
Met Eireann has forecast no let up in the rain.
Some 20-30mm fell overnight, mostly in parts of Connacht and the north-west, but another Atlantic system is expected to move in tomorrow and on into Saturday, dumping anywhere from 50-80mm of rain on the same region.
“There’s no sign of any dry weather yet,” forecaster Evelyn Cusack said.
Isolated rural homes in the lower Shannon below Parteen have been described as islands as engineers from both Limerick and Clare councils remain on hand to support locals.
Jim Casey, of the Office of Public Works (OPW), said flood gauges on the mid Shannon system around Athlone had recorded a six inch rise in the water over 48 hours.
“I would stress they are in a severe flood category and already rising from there,” he said.
Locals reported that the Shannon rose about 9cm from yesterday but the OPW said some feeder rivers, including the Suck and Brosna, were showing significant falls in flood levels.
The extent of the floods is expected to exceed 2009 and is also being classed as a worst in 20-year event.
But the worst has yet to hit parts of the country further down the Shannon.
Mr Casey added: “F lood forecasting models are indicating flood levels will continue to rise down to Lough Derg and Limerick and probably peaking early next week on Monday or Tuesday.”
The ESB, which has been criticised over its handling of the release of rising waters from the Parteen weir and into the old Shannon system, said it may have to increase the flow again.
The impact will hit hundreds of acres next to the Clare and Limerick border.
“We will be increasing the levels over Saturday and Sunday, again it will all depend on the rainfall over the weekend,” Tom Browne, ESB engineering manager, said.
Elsewhere, w ater levels in the Lee in Cork have been described as stable.
Sean Hogan, chair of the National Coordination Group which is overseeing the emergency flood response, appealed for the public to take extra care around flooded areas.
“We were concerned that complacency would set in. We think it’s important to try and avoid that,” he said.
In other regions the Shannon burst its banks in Carrick-on-Shannon, flooding the town’s retail park, Leitrim village was under water and the Suck remained in flood at Athleague, Co Roscommon.
The Defence Forces have deployed troops to at least six locations from Sligo down to Limerick and said they remain on standby to support flood defence and rescue efforts and provide off-road vehicles where needed.
The ESB has defended its control of water at Parteen, insisting that it can only release the water “when it is there”.
It said the dam has a small operating range which is insignificant faced with such a huge volume of water being drained out of the 10,000km sq S hannon catchment.
It takes four or five days for heavy rains in the upper and mid Shannon region to flow down to Parteen.