In total, 14,749 fish were found dead over a five kilometre stretch of the river Ollatrim in Co Tipperary.
The majority of the dead fish were lampreys (10,500), with brown trout (1,400), Stoneloach (805), Minnow (1,820), Salmon (70), Crayfish (70) and Stickleback (84) also among the dead.
Inland Fisheries Ireland immediately commenced an investigation following the discovery of the fish. Indications are that the fish kill occurred on Sunday 8 July.
This is the largest fish kill of Lamprey, a protected species, in recent years and it is anticipated that recovery will take several years.
The investigation to identify the source of the fish kill is continuing this week. The cause appears to have been a chemical agent, possibly a herbicide or pesticide, which has now passed through the system.
In the wake of the discovery, Inland Fisheries Ireland has advised the public and the farming community that if they are using spraying equipment to be aware that these herbicide and pesticide chemicals, even when diluted with water, are liable to be extremely toxic to all aquatic species.
Any mixing must be done far from natural watercourses, especially in the current conditions when diluting waters are in short supply, therefore increasing the toxicity of the chemical.
Inland Fisheries Ireland has a confidential hotline for the reporting of incidents by telephone on 1890 34 74 24 or 1890 FISH 24.