Swimming has been banned at three locations in South Dublin after seawater was found to be contaminated with high levels of the harmful bacteria E coli and Enterococci.
The temporary prohibition on swimming at Killiney Beach and Blackrock and Seapoint bathing areas will remain in force until Monday.
The prohibition warning was issued yesterday by Dun Laoghaire-Rathdown County Council after test results revealed the presence of the dangerous bacteria at the popular swimming areas.
Officials also issued warnings to pet owners to be aware that the poor quality of the water is not suitable for animals and that dogs should be kept out of the water at the three bathing areas until Monday.
The council initially issued warnings for Killiney and Blackrock and followed up with a second warning extending the swim ban to Seapoint.
Water quality in areas of Dublin Bay has been known to deteriorate after very heavy rain and the high level of rainfall in recent days is a suspected link to the problem.
However, a cause for the current contamination has not yet been announced.
Councillor Cormac Devlin, a Fianna Fail member of the council and former cathaoirleach, told RTE it was "alarming" that the ban was needed following the last prohibition notice issued in January.
He called for council staff to identify the cause of this latest contamination so that it can be addressed "once and for all".
"We are fortunate, in one way, that the weather is not forecast to be as warm and sunny as it has been," he said.
"However, the fact that this has happened yet again, for another summer, is alarming.
"Daily and regular swimming is common along our coast and I have asked the officials of the council to try investigate the root cause of this recurring problem."
Each local authority in Ireland is obliged to identify official swimming areas in their regions and ensure that their water quality is monitored for health and safety reasons.