Crèches, nursing homes and hotels using their own wells for drinking water, could pose a serious health risk.
A new report from the Environmental Protection Agency expresses concern about the standard of some private water supplies, and found the quality of drinking water in private supplies remains poorer than that in public supplies.
Small private supplies serve commercial or public buildings and are drawn from springs or wells -these have the poorest water quality.
One million people in Ireland - or almost 20% of the population - get their drinking water from private water supplies including household wells.
But this report says many of these supplies aren't registered with local authorities and so don't get tested properly to ensure the water is safe.
Last year E. coli - the contamination linked to human or animals waste - was found in 51 small private water supplies serving commercial buildings like hotels and B&Bs or public buildings like schools and crèches.
Meanwhile, no E. coli testing was reported for over 700 other small supplies - including 22 private supplies in Co Mayo.
The EPA is concerned about the health risks particularly for children and the elderly - and it's urging owners of all private water supplies to register with their local authority.
Owners are also urged to protect their private supply by checking the source of the water and checking for any sources of pollution, as well as testing the supply annually.