600,000 people in Dublin, Meath and Kildare have been told it's safe to use their tap water again.
An eight-day long boil water notice has been lifted.
It was put in place following problems at an Irish Water treatment plant in Leixlip.
Tests on the supply have shown that the water is now safe to use again.
However, Niall Gleeson, Managing Director of Irish Water, has warned the Leixlip plant "remains vulnerable".
For now, water will be diverted from the country's biggest treatment plant in Ballymore Eustace to ease pressure on the Leixlip plant.
"The ultimate solution is to replace the filters at the old plant", Niall Gleeson said.
"If we could, we would shut down the old plant, take it off line and do all the refurbishment at once".
However, that can't happen as the Leixlip plant supplies 20% of Dublin with its water and there is no back-up.