More than 800,000 people are being served by drinking water supplies where supply and quality is regarded as "at risk".
The figure, released by the Environmental Protection Agency, shows a rise of 21,000 since the end of 2015.
Among the main issues identified are inadequate treatment for cryptosporidium, ineffective disinfection and inadequate control of compounds that pose a risk to public health.
There are currently 119 water supplies on the EPA’s Remedial Action List.
The increase is partly attributed to the inclusion of two large supplies, in Whitegate in Cork and Loughrea in Galway, on the list.
The list is formulated as a result of EPA audits and investigations of drinking water quality failures.