The warning covers Dollymount, Sandymount and Merrion Strand
Members of the public in Dublin are being advised not to swim in some areas this weekend.
Dublin City Council has issued the warnings for Dollymount, Sandymount and Merrion Strand.
It follows a warning earlier this week after a power outage at the Spencer Dock Pumping Station, which Irish Water says resulted in wastewater building up and overflowing.
The utility said the incident appeared to have no impact on bathing water quality, but in a statement yesterday said: "Following advice from the HSE the Bathing Water Notices will remain in place at Dollymount, Sandymount and Merrion Beaches due to a second incident where suspected elevated levels of bacteria have been found in the wastewater discharge in the treated effluent from Ringsend Wastewater Treatment Plant."
Water samples were taken yesterday, with results due on Monday.
The warning comes amid the continuing warm weather in the city, with temperatures of up to 22 degrees being forecast for today.
Councillor Ciarán Cuffe has suggested the current system is not up to standard.
He observed: "Well I think Irish Water need to have a mechanism in place to have an early warning on possible pump failures. I think they need to have a back-up plan.
"I think it's ridiculous that the failure of one single pump can stop bathing all around Dublin Bay."
The Green Party Councillor added: "I'd say to the general public observe the warnings, and hopefully there'll be updates. With a bit of luck, we'll be back in the water very soon."