Drinking water in Co Dublin has been found to have 10 times the legal limit of lead.
Irish Water's test results show a further eight supplies across the country had above the permitted level.
The HSE's website states that lead may harm kidneys, contribute to high blood pressure and cause cancer.
According to details released under the Freedom of Information Act, nine of the 422 water supplies tested at properties across Ireland in the first six months of this year failed.
Three supplies in the Dun Laoghaire/Rathdown area had above the allowable limit with one more than 10 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 says the results are a concern.
"The problem with lead is, it builds up in a person's body over a long period of time," said Mr O'Sullivan.
"And the symptoms may not be obvious for quite some time.
"But most importantly, the people at the highest risk are very young children. Even unborn - the fetus in the womb - is at risk."
Irish Water says water that leaves its treatment plants is free from lead with most failures down to lead-plumping on people's properties.