The public is being warned that algal bloom may be present in the water and on the shoreline of Lough Leane in Killarney.
Kerry County Council's erected precautionary signage urging the public to exercise caution.
It comes as samples taken by the council in recent days, confirmed the presence of a blue-green algae called cyanobacteria, which indicates the potential for the formation of algae along the shoreline.
The public's advised not to bathe in the water if there are visible signs of algal blooms; they’re also advised that these blooms can be harmful to dogs, if ingested.
The council says it continues to monitor the situation.