Fingal County Council has issued a do not swim notice for Claremont beach in Howth, Co Dublin due to faecal pollution.
A routine sample taken on Monday showed that intestinal enterococci levels have been exceeded and the Health Safety Executive advised that swimming at the beach should be prohibited until levels return to normal.
A further sample will be taken on Thursday and sent for testing with the results due on Saturday,
Fingal County Council and the HSE have not yet determined the exact source of the incident but believe it is not related to wastewater infrastructure.
They are examining the possibilitythe contamination may have been caused by animals, birds or surface runoff.
The beach remains open but people are asked to not swim.
Members of the public intending to enter the water at any beach in Ireland are advised to check the water quality in advance by accessing the EPA website at beaches.ie or checking the notice board at the entrance to each beach.