Twenty-one bathing spots have been stripped of top marks for water quality because of last year's record wet summer, environment watchdogs have revealed.
Four of the country's 136 popular swimming areas were given a poor grade after failing to meet basic standards - Fountainstown in Cork, Rush in north Dublin, Ballyheigue in Kerry and Clifden in Galway.
The Environmental Protection Agency (EPA) said run-off from farmland sprayed with slurry and manure or saturated ground where cows, sheep and horses graze was mostly to blame.
Peter Webster, EPA senior scientific officer, said Ireland's results were similar to the UK where rains wreaked havoc with quality at 10% of beaches. "The fall is disappointing but I'm surprised it was not more. At times the rain was the equivalent of biblical proportions," he said.
The number of beaches and bathing places meeting stricter good standards dropped to 66.9% - 91 of 136 - compared with 83%, or 112, in 2011.
Out of 107 days in the May to September bathing season it rained for 80, the EPA said, with beaches in Cork and Kerry suffering the worst from Atlantic weather systems. Mr Webster said it is less of a problem on Mediterranean beaches because the sun kills off pollution caused by faeces. "Sunshine is the best disinfectant, and we barely saw it last year," he said.
Only Dun Laoghaire-Rathdown County Council, Sligo and Leitrim County Council had good quality at all their bathing spots - eight out of the total.
Under the EPA rules there is a 5% risk of picking up an illness or infection if you swim in water failing to meet the mandatory standards. "But you are probably at greater risk of catching a cold on a bus on the way," Mr Webster said. The EPA scientist said he does not expect 2013 to be as bad as last year.
Mr Webster also defended councils for closing beaches at risk of causing infections, including in Clare where bosses were heavily criticised for damaging tourism at Lahinch and Spanish Point last June. "If you get pollutants coming into those horseshoe bays and you've a westerly breeze, there's nowhere for it to go. And you don't need an awful lot of poo in the water to breach the limits," he said.
Clifden in Galway, which was closed to swimmers because of long-running problems with discharges from the nearby sewage works, is not likely to be up to standard until 2015. Fountainstown, at the mouth of Cork harbour, suffered from episodic but persistent pollution while Ballyheigue failed after one very uncharacteristic sample exceeding E. coli limits. In Rush, one bad sample for E. coli was blamed on a wastewater pumping station.