Crèches, nursing homes and hotels using their own wells for drinking water could pose a serious health risk.
The Environmental Protection Agency's latest report expresses concern about the standard of some private supplies.
Around one million people in Ireland get their drinking water from private water supplies - and many others regularly drink water from such small supplies.
However, the EPA report warns that many of these supplies aren't registered with local authorities.
That means they don't get tested properly to ensure the water is safe.
Last year, E. coli contamination - often linked to human or animals waste contamination - was found in 51 small private water supplies serving commercial buildings such as hotels and B&Bs or public buildings like schools and crèches.
Meanwhile, no E. coli testing was reported for over 700 other small supplies.
The EPA is concerned about the health risks particularly for children and the elderly.
The agency's Darragh Page said: "We would encourage all private water suppliers and local authorities to ensure that all private water supplies are on the register and are tested regularly.
"It is essential that all water supplies are tested to confirm that consumer’s health is not being put at risk."