A blocked sewer has left raw waste overflowing into a river that runs through St Anne's Park and Bull Island nature reserve for the past week.
Irish Water crews are working to fix the problem in what they have described as a "very complex" operation that will require an underground sewage tank to be emptied before the blockage can be reached.
Sewage has been entering the Naniken River through a culvert in Brookwood Rise, in Harmonstown.
The river then flows through St Anne's Park, along community playing pitches and the park's duck pond before entering the Bull Island estuary.
Dublin City Council (DCC) said it had been told of the incident by a member of the public on February 28 and it had been using sandbags to block off the inlet into the pond.
However, the river then flows into the estuary that serves Bull Island, where the protected salt marshes and dunes are home to 180 bird species, as well as rare plants and insects.
DCC said no deaths of fish or birds had been identified.
The Environmental Protection Agency (EPA) said Irish Water had notified it of the incident on Monday and that it was investigating.
"Our priority is to ensure that Irish Water completes the corrective actions needed to unblock the sewer, resolve the incident and protect the environment," it said.
Irish Water said it had notified Inland Fisheries Ireland.
"A blockage suspected to be located in a syphon under the river is the likely cause of this overflow," it said.
"It will be necessary to empty an underground sewage tank so that crews can access the blockage in order to remove it. The health and safety of the crews involved in this work is paramount.
"The situation is ongoing and very complex. It is not possible to divert the sewage. We are continuing to work with Dublin City Council to resolve it as quickly as possible."