The offending swimming areas were Sutton Burrow in north Dublin; Clifden, Galway; Lilliput on Lough Ennell in Westmeath; and Ballyallia lake in Ennis, Co Clare.
But the Environmental Protection Agency (EPA) said 97% of Ireland’s 122 seawater and nine freshwater bathing spots meet European Union (EU) minimum mandatory hygiene standards.
Nine out of 10 reached honours standards — up 8% on 2009.
EPA director general Dr Mary Kelly said last year’s dry summer and improved treatment plants in towns and cities have helped improve quality.
“However, stricter standards for bathing water will be introduced under legislation, which means that greater effort will be needed to ensure that our bathing waters are clean, and fit for the people of Ireland to swim in,” she said.
EPA experts said Clifden has had persistent water quality problems because of its waste water treatment plant.
Micheal Lehane, from the EPA’s office of environmental assessment, said: “It needs a new waste water treatment plant. There was some remedial work a few years ago but the council is aware.”
Although there were good marks for the vast majority of bathing areas, 11 locations risk not making the prestigious list of Blue Flag beaches judged by An Taisce and named in June.
Tests on these found traces of human or animal waste were found in 10 locations. They were Ballyallia, Lilliput and Clifden; Dollymount, Sandymount, Balbriggan front strand and Loughshinny, in the Dublin area; Ardmore in Waterford; Duncannon in Wexford; and Silver Strand in Wicklow.
Mr Lehane said Ballyallia has suffered as it has a catchment area near intensive farming while the authorities in Lilliput have created a reed bed near public toilets to prevent any further potential pollution.
The EPA’s bathing report found six spots suffered a drop in quality on 2009, including Ballyallia lake, which went from sufficient to poor quality and then five others which went from good to sufficient — Fountainstown in Cork; Inny at Waterville, Co Kerry; Portnashangan on Lough Owel and the Cut on Lough Lene, both Co Westmeath; and Clogga Beach in Wicklow.