Some 20,000 people have been warned not to drink mains tap water after dangerous levels of bugs were detected in supposed treated water.
Minimum safety standards are not being met in some water treatment plants throughout the country, a report from the Environmental Protection Agency (EPA) warns.
Alarming levels of bugs, including e-coli and cryptosporidium, have been detected.
As a result, notices advising people to boil their water have been issued in parts of Kerry, Roscommon, Tipperary, Leitrim, Waterford, Mayo, Wexford and Longford.
Such notices are only put in place by local authorities after consultation with the HSE which deems the supply to be dangerous to public health.
About 940,000 consumers receive water from treatment plants which need to be upgraded to “secure” quality levels. However, 20,000 are currently drawing supplies from contaminated sources, and this water is considered “unsafe” to drink.
While many people who drink untreated water can suffer some side effects, most will not require medical attention.
“Some plants are very old, and because of their condition cannot be fully relied on.
“If there’s cryptosporidium in the water, there’s the potential for someone to become ill,” said Jerry Grant, Irish Water’s head of asset management.
“At the moment, that particular problem affects about 20,000 people. They’ve been put on boiled water notice.
“There’s also a potential for cryptosporidium breakthrough.”
Out of 140 treatment plants, 14 have so far been brought up to up World Health Organisation (WHO) and EU compliance standards.
A further 10 are currently with the EPA for testing before approval is granted to remove them from the list.
But he insisted measures are well underway to rectify these problems.
Six individual water treatment plants are being built across Roscommon, and will be commissioned between November 20 14 and May 2015.
“That will take the bones of the 20,000 currently affected off boiled water notice,” he told the Irish Independent.
Part of the problem is that treatment plants were not expanded and upgraded to meet the huge increase in house building during the boom.
In 2012, a total of 47 boil water notices – affecting 43 supplies – were put in place affecting 50,000 consumers.