Meath County Council has said a discharge of sludge from a sewage treatment plant at Oldcastle caused a big fish kill on the River Inny in Co Meath.
Up to 4,000 wild brown trout were killed following the discharge.
A council spokesperson said sludge was released following a malfunction at the plant.
Efforts are continuing to save remaining stocks on the river and nearby Lough Sheelin.
The Shannon Regional Fisheries Board has said it will take years to replenish the river.