Swimmers are being advised to stay away from one of the capital's most popular bathing areas.
Blackrock Baths and Seapoint in south Co Dublin have been hit with an E.coli outbreak.
Dun Laoghaire/Rathdown County Council regularly monitors the water quality along the coast.
However, samples taken from both locations last Monday indicate a high level of E.coli. The local authority has taken the precautionary steps of placing a temporary prohibition notice, advising people against swimming.
The cause of these high levels are not known and the council is investigating. New samples have been taken and are expected back on Monday.
In a statement, the council said: "If the re-sample results are acceptable, the temporary prohibition notice will be removed."
E.coli (Escherichia coli) are bacteria found in the environment, foods and intestines of people and animals. Contamination allows the organism to spread to water environments.
Cormac Devlin, the Cathaoirleach of Dun Laoghaire-Rathdown County Council, said: "Thankfully the samples taken have uncovered this particular outbreak of E.coli along the coast at Blackrock and Seapoint.
"Hopefully it's an isolated incident and I'm advised that the results will be known by next Monday. In the meantime the public are asked, especially swimmers, to stay out of the water for their own safety.
"Dog owners are reminded of the effect on them also. Notices have been put up at these locations."