Almost 10% of Ireland's public water supplies are at risk of contamination, according to the Environmental Protection Agency’s Drinking Water Report.
The EPA says remedial action is needed at 87 separate "at risk" sources to safeguard water quality for 700,000 people.
Counties Kerry, Cork, and Donegal account for almost half of the "at risk" supplies.
The EPA points out that action programme dates set out by Irish Water to improve 24 "at risk" supplies have slipped.
Fifty-eight of the "at risk" supplies have elevated levels of trihalomethanes - a by-product of the chlorination process.
Twenty-five lack adequate treatment to prevent Cryptosporidium, a parasite found in human and animal waste.
However, the report says that, overall, the quality of water in Ireland remains high and the incidence of microbiological contamination continues to fall.
Over 140,000 water test results taken from 900 public water supplies throughout Ireland were analysed for contaminants by the EPA last year.
Overall the results were very good - 99.9% passed the microbiological standards, while 99.5% passed the tests for chemicals.
The presence of bugs that can cause people to fall ill such as E Coli and Enterococci continued to fall.
Four thousand fewer people were on boil water notices at the end of 2016 compared with 2015.
Irish Water said the EPA report clearly demonstrates that communities across the country are continuing to benefit from its ongoing strategy, investment, and implementation programmes in safeguarding drinking water.