Tipperary County Council has warned of the presence of an algal bloom on Lough Derg that can produce toxins.

A statement from Tipperary County Council, issued in consultation with the Health Service Executive (HSE) on September 23 said: “The blue green algae bloom can produce harmful toxins and is most easily recognised when it forms a bright pea-green paint-like scum on or close to the shoreline.

“Contact with this bloom should be avoided as it may cause skin irritation.”

Additionally, the public are advised to keep all animals, especially dogs, away from the affected area.  

Tipperary County Council also advised the public to ensure that animals have an alternative source of drinking water.

Algal bloom

According to a Dublin City University (DCU) study: “Freshwater harmful algal blooms (HABs) are becoming more frequent, persistent and widespread as climate change, eutrophication and land-use change accelerate.”

Bacteria can release microcystin-LR (MC-LR), threatening drinking-water supplies, recreation, aquaculture and ecosystems.

The DCU study said that events such as the Lough Neagh bloom in 2023 demonstrate the costs of delayed detection:

Prolonged recreational-site closures can reduce tourism and local trading;
Remediation companies may repeat treatment or mobilisation while awaiting verification;
Water utilities may increase sampling, alter treatment or restrict abstraction without timely toxin evidence.

It added: “These operational costs sit alongside distinct regulatory and public-health decisions, which may require accredited confirmation.”  

Lough Derg

Almost exactly a year ago (September 25, 2025) an outbreak of algal bloom was reported in Lough Derg.

Then, as now, the local authority advised the public to exercise caution, adding that contact with the bloom should be avoided as it may cause skin irritation, and to ensure alternative sources of drinking water for animals.

Members of the public with concerns on the issue can contact Environment and Climate Action Section of Tipperary County Council at 0818 06 5000.