25 drinking water supplies around the country are at risk of being infected with cryptosporidium.
Among them are 13,000 people drinking inadequately-treated water in parts of county Sligo served by the Lough Talt supply.
However, the Environmental Protection Agency says more than 99 per cent of drinking water samples complied with safety standards last year.
But in a new report, it says 87 supplies around the country need to be upgraded to avoid the risk of contamination by chemicals or infections.
700 thousand people are served by these 'at risk' supplies, with nearly half of them based in Cork, Kerry and Donegal.
Darragh Page from the EPA says a big problem in many supplies is the risk of contamination with THM chemicals.