The European Commission took the case due to the levels of trihalomethanes in drinking water - which are used to remove bacteria.
Europe's highest court says drinking water in Ireland exceeds safety levels.
The Court of Justice says the state failed to meet its obligations on directives.
The court found levels of certain chemical compounds in Irish tap water are both damaging to human health and to the environment.
The European Commission took the case due to the levels of trihalomethanes in drinking water - which are used to remove bacteria.
They can also cause stomach problems and skin irritation.
The case dates back as far as 2014 when Irish Water - now Uisce Eireann - had been asked to ensure drinking water met standards.
Today's findings relate to 21 public water supplies and nine private group schemes.
Local TD Accepts Data Centres Are A "Particular Issue" In Ireland
Katie Taylor Tickets Sell Out for Croke Park Final Fight In September
New Digital Wallet Might Be Needed To Access Pornography Online
Two Men Arrested As Gardaí Seize Firearm And Drugs In Blanchardstown
Trial Of Man Accused Of Attempted Murder Of Three Children Will Be Shown CCTV Today
Second Night Of Riots In Belfast
Victim Loses Eye After Attempted Murder In Belfast On Monday
Report Finds Over 95,000 Children Are In Poverty
