Residents of Corofin in Co Clare have been told not to use or drink tap water due to the presence of algae in the supply at Inchiquin Lake.
The alert affects up to 1,800 customers in the Corofin and Ruan Public Water Supply.
The algal bloom follows a long period of dry weather, warm temperatures and reduced water levels in the lake.
Residents have been told that the water supply is unsafe for use until further notice except for flushing toilets and laundry.
Lake users and farmers with lands bordering Inchiquin Lake have also been advised not to bathe in the lake or allow animals to enter the water.
The Health Service Executive, Irish Water and Clare County Council also said people should not use ice cubes.