Hundreds of fish were found dead in the River Liffey upstream from a reservoir that supplies drinking water to Dublin.
The find has prompted concerns and investigations are under way at the site two kilometres from the Poulaphouca Lake reservoir in Co Wicklow.
A fisherman discovered several dead fish including trout, salmon, and a number of other species in the water last week.
Wicklow County Council and Inland Fisheries Ireland are currently investigating water samples and fish remains to find the cause and exact location of the incident.
A spokesman for Irish Water said that initial investigations confirmed that that the fish kill wasn't caused by Irish Water infrastructure or operations.
He added: "There is also no impact to drinking water supplies in the Greater Dublin Area as all water testing results are clear with regular monitoring in place.
"Further detail may be available from the Inland Fisheries and Wicklow County Council Environment Section."
A spokeswoman for the EPA said: "The EPA has no licensed facilities in the vicinity of the fish kill, and there have been no wastewater treatment issues in that vicinity either."
