A report on water quality has found that thousands of people took supply from group schemes contaminated by E.coli.
The report by the Environmental Protection Agency cited 14 private group schemes that had signs of E.coli bacteria.
The agency warned that urgent action is needed to improve drinking water quality provided by private schemes and supplies in rural areas.
It found that overall, private drinking water quality is not as good as public water quality.
In its latest report it identified several issues and the possible dangers it is causing to public health.
It highlighted the failures across 14 private group water schemes to meet basic disinfection requirements, with an E.coli contamination detected.
It warned that this put the health of around 5,500 people at risk.
Drinking water is provided to approximately 200,000 people across rural communities in Ireland, but the total number of small private supplies remains unknown as not all have registered with their local authority.
Director of the EPA's Office of Environmental Enforcement Tom Ryan said since there there is no legal requirement to register private drinking water supplies, it is not possible to quantify the full extent of the risk to public health.
He said: "It is a local public health concern that private drinking water quality hasn't improved in recent years despite the availability of public funding to support upgrades to water supplies."
