Homes on Dublin's southside are discharging raw sewage directly into the sea which is affecting water quality, the Environmental Protection Agency (EPA) says.
The watchdog believes homes have been improperly connected to storm water drains instead of the sewer network, resulting in effluent being discharged into streams without treatment before entering Dublin Bay.
The findings are contained in the 2017 Bathing Water Quality in Ireland report published today.
It says that of "several hundred" properties visited around the Trimleston and Elm Park streams, one in 12 had "misconnected drains".
The EPA said the raw sewage, coupled with bird droppings and bacteria being dredged up by the tides, has resulted in Sandymount Strand and Merrion Strand being classed as having "poor" quality water, meaning they are unsafe for swimming. Sources suggest businesses may also have been improperly connected, and the overloaded sewerage treatment plant at Ringsend, which is in the process of being upgraded, was also resulting in problems.
The EPA report finds that seven bathing waters are classed as "poor", five in Dublin and two in Galway.
In most cases, new wastewater treatment plants are needed to improve quality.
They include Clifden in Connemara, where water quality results are described as a "major disappointment" as Irish Water had completed a new wastewater treatment plant in 2015.
However, investigations by the local authority suggested there were "several additional pollution sources" and concerns about how the new plant was operated which was leading to quality issues.
"Improved operations" coupled with reducing discharges from storm water drains, which were affecting quality, were also required.
Irish Water said it recognised the "urgent need" to address locations where untreated wastewater entered rivers and seas, with an average of €326m to be spent every year on wastewater infrastructure.
"Irish Water has made a €7.75m investment to address significant inadequacies in the wastewater treatment infrastructure in Clifden," it said.
"The Clifden sewerage network was upgraded and the construction of a new wastewater treatment plant for the town was completed in October 2015. Irish Water is currently working with Galway County Council to investigate the causes of bathing water failures."
The report finds that where investment is made, it does lead to improvements, citing a new plant at Ardmore which resulted in higher water quality, along with notable improvements in Youghal Front Strand due to investment.
However, it warned: "Much of the infrastructure investment required to rectify issues with wastewater discharges features in Irish Water's capital investment programme...in most cases, the time frame is several years off due to the conflicting priorities of balancing investment in other areas of drinking water and wastewater infrastructure."
EPA programme manager Andy Fanning said urban beaches were under greater pressure and that more needed to be done to "eliminate the sources of bacterial contamination that are particular to urban locations" including misconnections and sewage discharges.
Irish Water has ruled out using sea water as a new drinking supply for Dublin and the midlands, saying it would require six times as much energy to make the water drinkable and produce millions of litres of 'salty' brine which would have to be disposed.