Residents in parts of Co Galway have been hit by a new outbreak of the cryptosporidium parasite.
Tests on the Roundstone Public Water Scheme showed trace elements of the parasite, as did water schemes for Inishnee and Errisbeg.
Between 300 and 350 houses are believed to be affected.
No illnesses have been reported yet, but Galway County Council has instructed residents in these areas to boil tap water before using it for drinking, brushing teeth or food preparation.
A spokesman for the council said the supplies did not comply with drinking water standards, and that the council was looking at possible options, temporary and long term, to improve the quality of supplies. Proposals are expected to be submitted to the Department of the Environment Heritage & Local Government before the end of the week
"The presence of cryptosporidium in the water supply indicates a failure of the water treatment process to screen it out and remove it," the council said in a statement.
"The council is assessing the existing treatment plant to see if measures can be introduced to improve its effectiveness. However, the water supply for the Roundstone area is drawn from a source that is regarded as vulnerable and the optimum solution is to bring an alternative piped water supply to the area."
The outbreak comes shortly after Galway City Council said water supplies had been restored to the west of the city and part of the county.
Cuts had affected up to 20,000 people after local electricity outages affected pumping from Terryland waterworks to a reservoir at Tonabrucky, resulting in the reservoir almost running dry.