Thousands of homeowners along the Shannon are fearing devastating floods after the river reached record levels.

Energy chiefs at the ESB were forced to open gates on dams increasing water flow by 10% as they monitored exceptional flooding upstream.

Levels on Lough Derg were the highest ever with Athlone also suffering as the entire Shannon system swelled.

"The amount of water coming in and speed at which it is coming in hasn't been seen before," an ESB spokesman said. "We can't empty it as fast as it's filling,"

Limerick city, and towns, villages and farms on floodplains downstream from the Parteen weir near the Clare-Tipperary border are expected to bear the brunt.

The Parteen Weir controls water flow into Ardnacrusha power station further downstream.

Councils in Clare, Limerick and North Tipperary and local emergency services have all warned areas from Parteen to Limerick city to expect further floods overnight. A small number of homes have been evacuated. The worst affected areas are expected to be Montpellier and Castleconnell.

Taoiseach Brian Cowen visited the worst flood-hit areas of Cork, Ennis, Clonmel and Galway county, where waters have begun to recede opening roads for the first time in three days.

"There's a huge effort being made here to mitigate the worst aspects of this. That's very heartening," Mr Cowen said.

The Taoiseach said funding for flood protection was a priority.