A boil water notice has been issued for parts of Wicklow and south Dublin, affecting around 65,000 people.
It has been issued as a result of the mechanical failure of a chlorine booster at the Vartry Water Treatment Plant.
Irish Water said it will work with the Health Service Executive to try to remove the boil water notice as soon as possible.
The company is carrying out chlorine dosing on the supply and has put in place a water sampling programme to test the chlorine levels in the affected areas.
In the meantime, all customers of this supply are advised to boil water before use until further notice.
The notice affects the following areas in Co Wicklow:
Ashford, Newcastle including Newcastle Hospital, Newtownmountkennedy, Kilcoole, Kilquade, Kilpedder, Kilmacanogue including Glenview Hotel/Kilmurray Cottages, Greystones, Cookstown Road, Fassaroe/Berryfield Lane, Kilcroney, Delgany and Bray.
A number of areas in south Dublin are also included:
Corke Abbey, Woodbrook Glen, Old Connaught Avenue, Thornhill Road, Ballyman Road, Ferndale Road from its junction with Old Connaught Avenue to Allies River Road, the Dublin Road from the junction of Old Connaught Avenue to Allies River Road, and all areas off these roads.
In a statement, Irish Water said that water must be boiled for drinking, food preparation, brushing teeth and making ice.
It said that domestic water filters will not make water safe to drink.
