Drinking water in Co 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 8 supplies across the country had above the permitted level.
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.
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.
Irish Water says water that leaves its treatment plants is free from lead - with most failures down to lead-plumping on people's properties.
According to the HSE's website, lead may harm kidneys, contribute to high blood pressure and cause cancer.