A number of bathing areas on Tipperary's Lough Derg shoreline have been declared unfit for bathers because of concentrations of algal bloom.
The bloom was detected following routine inspections of the lake by Tipperary (North Riding) County Council. Initial results have shown the presence of blue-green algae.
Mr Marcus O'Connor, senior environmental engineer with the council, said that notices had been posted at lakeside resorts advising that the water was unfit for bathing because of concern over the algae.
The council is waiting for further test results to assess the toxicity of the algal bloom. However, it is confident it is blue-green algae.
"We are also advising people to keep livestock and animals away from the lake. We have posted signs and we are monitoring this on a continuous basis," added Mr O'Connor.
People are advised not to swim in the lake or drink water from it. However, the council said Nenagh's drinking water, taken from the lake, was of a satisfactory standard.
Tipperary (North Riding) County Council notified counterparts in Galway and Clare of the situation.
Last year similar signs were posted on the Tipperary lakeside for a number of weeks towards the end of the summer.
Algal bloom suffocates fish after entering their gills and can be poisonous to people. It usually occurs in warm, dry weather and it is caused by the use of fertilisers and sewage. It leads to an excess of nutrients in the water.
Farming continues to be the largest single source of lake and river pollution.