A drinking-water supply in Co. Dublin has been found to have 10 times the legal limit of lead - which is believed to be linked to cancer.
Irish Water's test results show three supplies in the Dun Laoghaire-Rathdown area had above the allowable limit - it also included the supply which was worst performing in the country.
A further eight supplies across the country had above the permitted level.
According to the HSE's website, lead may harm kidneys, contribute to high blood pressure and cause cancer.
According to details released under the Freedom of Information Act, of the 422 water supplies tested at properties across Ireland in the first six months of this year, nine failed.
A water supply in the Dublin City Council area had five times above the limit, while there were also failures in Longford, Tipperary, and Galway.
Environmental consultant Jack O'Sullivan says the results are a concern. "The problem with lead is, really it builds up in a person's body over a long period of time," he said.
"And the symptoms may not be obvious for quite some time. Most importantly, the people with the highest risk are very young children. Even unborn, the foetus in the womb is at risk if the mother has been exposed to lead."
Mr. O'Sullivan believes the State should try and repair the issues. "It's such a small number, that it would not be costly for the State to simply to say to the people living in those areas 'we're going to replace all your lead pipes for you'," he added. "And it would be well, well worth it. Because, clearly you would be saving peoples - not just their lives, but lead in very low levels affects people's general behaviour and the way they think."
Irish Water says water that leaves its treatment plants is free from lead - with most failures down to lead-plumping on people's properties.
