Tipperary County Council is warning of the presence of Algal Bloom on Lough Derg.
The algae which has been observed in recent days can produce harmful toxins.
The bloom is most easily recognized when it forms a bright pea-green paint like scum on or close to the shoreline.
The public are being advised to exercise caution – contact with this bloom should be avoided as it may cause skin irritation.
All animals – especially dogs – should be kept away from the affected area.
Farmers are being told to ensure animals have an alternative source of drinking water.
The Environment & Climate Action Section of Tipperary County Council can be contacted on 0761 06 5000.