A boil water notice is in place in Glencullen and Ballybetagh in south Dublin.
It follows the discovery of the bacteria Giardia in a routine sample test.
Irish Water has advised 156 households and businesses in the area today to boil water before using it until further notice.
It said its experts are working with colleagues in Dún Laoghaire-Rathdown County Council to resolve the situation as soon as possible.
Irish Water has said it cannot yet say when the notice will be lifted.