A boil water notice is in place in Fintown, Co Donegal, following the discovery of Cryptosporidium in the water supply there last week.
Irish Water says around 450 customers are affected by the discovery of the parasite in the Fintown water treatment plant.
The boil water notice applies to both the Fintown public water supply and the neighbouring Meenmore-Meenatinney group water scheme.
Irish Water said last Friday that it would be contacting "vulnerable" customers by telephone and that they would be distributing door-to-door notifications in the area.
The very young, the elderly and sick people are particularly vulnerable to respiratory and gastrointestinal illness caused by drinking water contaminated by Cryptosporidium.
It is unclear when the boil water notice will be lifted as tests on the water supply are ongoing.
The principal of Fintown National School says that the boil water notice is badly affecting the running of the school.
Shane Mac Claifeartaigh said the school has turned off its own water supply for health and safety reasons.
