Drinking water in County Dublin has been found to have 10 times the legal limit of lead - which is linked to cancer.
Irish Water's test results show 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, Irish Water tested the water supply at 422 properties in the first six months of this year – with nine failing.
Three supplies in the Dun Laoghaire/Rathdown area had above the allowable limit – with one more than ten times above it.
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 said the results are a concern.
“The problem with lead is really it builds up in people’s bodies over a long period of time and the symptoms may not be obvious for quite some time,” he said.
“But most importantly, the people at the highest risk are very young children.
“Even the unborn, the foetus at the womb is at risk if the mother has been exposed to lead.”
Irish Water says water that leaves its treatment plants is free from lead - with most failures down to lead-plumping on people's properties.
Mr O'Sullivan called for immediate action to replace affected pipes.
“It is such a small number that it would not be costly to the State to say to the people living in the areas, ‘we are going to replace all your lead pipes for you.
“And it would be well, well worth it because clearly you would be saving people’s lives
“Not just their lives because lead in very low levels affects people’s general behaviour and the way they think.”
Reporting From Eoghan Murphy