An algal bloom has affected the Vartry water-treatment plant at Roundwood, Co Wicklow, resulting in supply being cut off or reduced overnight to a wide area of northeast Wicklow.
Irish Water said the bloom, which blocks filters, has reduced capacity at the Vartry scheme from 65 million litres to 40 million litres per day.
As a result, the reservoir serving Enniskerry is “badly affected”, with no water available for a number of areas including Kilmolin, Kilgarron, Parknasilogue, Eagle Valley, Enniskerry village, Knocksink, Ballyman road, Monastery area, the R117 road and Millfield.
Irish Water said supplies to the wider Greystones area were being cut off from 8pm to 6.30am in a bid to conserve water. Adjacent areas affected include Windgates, Delgany and Killincarrig.
The authority warned households and businesses that it could take a number of hours beyond 6.30am for pressure to build up and supplies to be restored.
While the Vartry reservoir also supplies the Stillorgan reservoir in south Co Dublin, Irish Water said there was no danger to supplies there as Stillorgan was also connected via a reservoir in Saggart to the Ballymore Eustace reservoir.
However, a number of restaurants in north Wicklow have expressed fears for their businesses and for the Greystones Gourmet event this weekend.
Irish Water said water tankers have been deployed in a number of areas across northeast Wicklow, including Enniskerry and Wingfield Farm and Glencormac near Kilmacanogue.
Tankers are also serving Bellevue Hill and Kindlestown Rise in Delgany, Foxes Meadow and Tooman Road at the Willow Grove, Tinnapark in Kilpedder and Timmore Lane, south of Newtownmountkennedy.