The sewage pollution photographed near Ringsend in Dublin last month was caused by overload rather than a mechanical failure at the wastewater treatment plant according to the Environmental Protection Agency.
The EPA's investigation found that the amount of sewage entering the Ringsend plant had doubled in recent months and it was unable to cope.
The Ringsend Wastewater Treatment Plant was already running at 20% above capacity and is not complying with EU regulations on effluent discharge.
The plant caters for 40% of the country's wastewater.
In a statement an EPA spokesperson said there was a mechanical breakdown at the plant on Saturday 23 February but following a site inspection they added "the inspectors concluded that the incident was not the main underlying cause of the brown coloured plume, which was photographed by a drone on the weekend of the incident".
The statement added that it is likely that discharge problems will continue at the overloaded plant until upgrade works are completed between 2020 and 2023.
The EPA carried out water testing three days after the incident and found that the discharge was "slightly" over emission limits.
Testing at Dollymount Strand - the nearest bathing area - found that the water quality on the following Tuesday was within the "excellent" category.
But the statement adds "water quality in the area between the North and South Bull Walls is likely to be of poorer quality".
Results of samples taken on 1 March are not yet available.
It said repairs to the tank that broke down are expected to take one month and the shutdown of this section is causing a reduction in treatment capacity of up to 8% at the already overloaded plant.
Irish Water said that there have been two previous unplanned discharges at the plant in the past five years.
One was due to a mechanical breakdown during heavy rain in June 2017 and a power failure in March 2018 caused another uncontrolled release.
There was a planned discharge during upgrading works in November 2018 and there are overflows classed as "authorised discharges" during heavy rainfall such as occurred during Storm Brian in October 2017.
Irish Water says it is investing €400 million in upgrading the Ringsend plant which will allow it to cater for an additional 400,000 population equivalent.