Water tankers are being brought in for around 1,800 customers of a public water supply in Co Clare.
The Corofin and Ruan Public Water Supply source has been deemed unsuitable for drinking due to the presence of algae.
Water customers in Corofin town and its environs and in Ruan village, as well as customers of the Killeen Group Scheme Supply are affected by the alert.
The HSE, in conjunction with Irish Water and Clare County Council, is advising customers of the scheme that tap water should not be used for drinking.