An investigation is under way by Inland Fisheries Ireland (IFI) after the Dodder Valley sewer overflowed into the river in Firhouse last night.
Uisce Éireann said the sewer overflowed near Firhouse Weir following heavy rainfall and thunderstorms over a short period of time.
IFI said it became aware of the overflow early this morning and conducted a number of inspections of the site.
In a statement it said "when IFI staff arrived, the discharge into the water had ceased, and there was no discernible impact on fish mortality".
Uisce Éireann said it has been liaising with South Dublin County Council, the EPA and Inland Fisheries Ireland in relation to the overflow.
A statement issued this evening by Uisce Éireann said: "The unprecedented nature of the storm and the sheer volume of rainfall in a very short period of time caused a release on our network".
It said a clean-up is being carried out in the area by South Dublin County Council.
Local Green Party Councillor Lyn Hagin Meade was on her way home from a South Dublin County Council meeting yesterday evening when she came upon the flooding.
She told RTÉ News "the situation was impassable" and said she was concerned about the impact on aquatic life along the Dodder River.
Cllr Meade said "as climate change makes events of flash flooding more common, we must do all we can to proactively prepare and mitigate blockages and contamination".
