A boil water notice has been issued for all areas of North Wicklow.
The notice was issued as a result of a mechanical failure of a chlorine booster at the Vartry Water Treatment Plant.
In a statement, Irish Water have said that water must be boiled for drinking, food preparation, brushing teeth and making ice.
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.
Roughly 65,000 people will be affected by this notice.
Irish Water will liaise with the HSE with a view to having the Boil Water Notice removed as soon as possible.
The company are carrying out chlorine dosing on the supply today and have put in place a water sampling programme to test the chlorine levels in the impacted areas.
In the meantime, all customers of this supply are advised to boil water before use until further notice.
Irish Water have added that domestic water filters will not make water safe to drink.