The Environmental Protection Agency is investigating the discharge of thousands of tonnes of raw sewage flooding into Dublin Bay.
Irish Water says its teams are working to fix a failure at one of its tanks in Ringsend, which spewed sludge into the sea on Saturday.
Works to upgrade the treatment plant started a year ago to try and meet demand from Dublin's growing population.
Irish Water said in a statement that in order to treat the increasing volumes of wastewater arriving at the plant to the required standard and capacity, it is investing over €400m to upgrade the Ringsend plant.
Currently, the Ringsend treatment plant treats approximately 40% of the country’s wastewater load.
Works started in February 2018 and are progressing well, Irish Water said.
However, Green Party leader Eamon Ryan says it is not acceptable that the public were not told to stay out of the sea at the weekend.
"Even at this time of year, even with the cold water out there, Dubliners are swimming in the Bay," he said.
"Why did Irish water not tell people to get out of the water for the weekend because they've got all this sewage in the bay, that's the real question they need to answer, as well as what went wrong."