Further boil water notices for the Greater Dublin Area have not been ruled out by the Environmental Protection Agency.
The warning echoes comments made by Irish Water last week and follows the EPA's latest audit of the Leixlip Water Treatment Plant.
A boil water notice affected 600,000 people in parts of Dublin, Kildare and Meath in recent weeks.
It was announced because of issues with the quality of supply from the Leixlip plant.
The EPA said that until filter upgrade works are complete in Leixlip, operational difficulties are likely to continue there.
The agency expressed concern over the ability of the plant to maintain and sustain an appropriate level of performance.