A boil water notice has been put in place in Co Mayo, after cryptosporidium was detected in the Knock Airport water supply.
Businesses, and a small number of homeowners, getting their water from this source have been told to boil it before use, until further notice.
The warning also applies to the Cloonlyon Group Water Scheme along with Cloonlyon National School and approximately 15 households.
Irish Water says it is working to resolve the problem.
In the meantime, people are being advised to boil any water used for drinking, food preparation, ice, or brushing teeth.