Raw sewage has been discharged into the Irish Sea for up to 26 hours because a malfunction went unnoticed at a treatment plant.
A member of the public on Balbriggan Beach in north Dublin raised the alarm on Tuesday afternoon when the waste water was washed back in with the tide.
Fingal County Council has closed the beach to swimmers and is awaiting results of water-quality samples from Balbriggan and nearby Barnageera and Skerries, which are due on Thursday.
"It is not possible to accurately measure the exact overflow volume discharged during the outages," said a council spokeswoman.
"We are treating this incident very seriously."
Council chiefs said Isaac's Bower pumping station in Balbriggan was out of action for 26 hours after extensive power interruptions in the area on Monday on Tuesday.
An automatic warning alarm was not activated as the power cuts also caused an item of the electrical control equipment to fail.
When the treatment plant filled with sewage it overflowed into the normal water drainage system and was then discharged into the sea, the council spokeswoman said.
"During this time there was extensive heavy rain in the area, adding to the volume of water being carried in our drainage network and the consequent overflow volume coming from the pumping station," she said.
"This is what caused the sewage overflow on Balbriggan Beach yesterday afternoon."