The Environmental Protection Agency has raised concerns about the quality and monitoring of drinking water standards in private supplies around the country.
It called for a greater effort to ensure supplies are fit for consumption.
Around a million people get their drinking water from a private supply.
Today's EPA report covered 417 private group water schemes, which are typically operated by local communities.
It also examined standards in small private supplies, serving places such as pubs, restaurants, hotels, schools and nursing homes.
Well over 90% were compliant with monitoring obligations and quality standards but 20 private group water schemes and 88 private supplies failed to meet bacteria controls.
The EPA said this is a matter of "significant concern" given the potential for illness as a result.
In addition, almost 20% of the country's 1,750 small private supplies had no monitoring in 2019, meaning it is impossible to be confident that the water from them is safe for consumption.
The report did not deal with 180,000 private wells.
The EPA said that suppliers, county councils and the Department of Local Government need to ensure standards are improved and maintained.
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Speaking to RTÉ's Morning Ireland, a senior EPA inspector said the report found over 100 private water supplies in 2019 contained bacteria that can make people ill.
Dr Michelle Minihan said failures in quality can arise for a variety of reasons and she urged water suppliers to carefully monitor their water and have it regularly tested.
If any problem is identified, she said, then the supplier should take action to protect human health.
She said that over a fifth of registered small private supplies were not monitored at all in 2019 and it is impossible to say if these supplies are safe.
Dr Minihan said the responsibility for oversight rests with local authorities who have a register of private water supplies and this register should be kept up to date, while local authorities should also ensure supplies are tested annually.
In addition, she said, the Department of Housing, Local Government and Heritage has developed a remedial action list for group water schemes from 2016 which is a list of schemes that require upgrades.
Overall, 21 out of around 106 schemes have had their upgrade works completed and another 58 have begun improvement works.
This, she said, shows that some progress is being made.