The Environmental Protection Agency has said it is particularly concerned about risks to public health posed by poor quality drinking water from private water supplies.
It said that many such supplies are not registered or monitored sufficiently to ensure safe drinking water and that E. coli contamination has been found in 51 supplies tested.
One million people in Ireland get their drinking water from private supplies.
Many more drink unknowingly from those supplies in places such as hotels, bed and breakfasts, pubs, schools, crèches, and campsites.
These supplies are mostly in rural areas and provide drinking water to people who are not connected to the public water mains.
The water source in most cases is a spring or a well.
However, the EPA said that many private supplies are not on the local authorities' register and those that are registered are not monitored sufficiently to ensure safe drinking water.
It found E. coli contamination in 51 small private water supplies serving commercial buildings and public buildings.
It is also particularly concerned that no E. coli testing was reported for another 711 small water supplies.
The EPA said this poses a serious risk to public health, particularly to vulnerable people such as the young and elderly.
Consuming water with E. coli in it generally leads to gastrointestinal illness, but in a small number of cases can result in severe and long-term kidney failure.
The EPA said those responsible for private water supplies need to ensure their supply is on the local authority register and is tested at least annually.
It also wants local authorities to properly police and test the supplies and to make sure action is taken where contamination is found.
The Minister for Housing, Planning and Local Government has called on people with private water supplies to register with their local authorities.
Eoghan Murphy said the registering of private schemes would help to improve the monitoring and quality of the water supplies.
The Programme Manager of the EPA's Office of Environmental Enforcement said 20% of the population are served by private water supplies.
Speaking on RTÉ's Morning Ireland, Darragh Page said: "Although the supplies serve 20% of the population, most of us are affected by them at some stage.
"We've all visited hotels in rural areas or had our children in créches, some of which may have their own wells. So we are all affected by this, directly and indirectly, so it's important that we are aware of this issue.
"Generally speaking younger children, the elderly and those who are already ill for other reasons - those are the ones who are most at risk."