People are being advised not to swim in the sea at South Strand Skerries due to E. coli in the water.
A temporary advisory notice was issued by Fingal County Council after preliminary test results showed the presence of the bacteria at higher than acceptable limits.
The sample of water was tested yesterday.
The council said the likely cause for the higher than normal levels of E. coli was the "impact of animal and birds as well as contamination of urban surface waters".
In a statement it said the higher levels were normally short-lived and that bathers were unlikely to experience an illness.
It said the full results, including enterococci rates, are due tomorrow and a resample has also been scheduled for tomorrow.
The council said beach should avoid swallowing or splashing water, wash hands before handling food, avoid swimming with an open cut or wound and those who are pregnant or have weakened immune systems should avoid swimming.