Twenty-three of the country's beaches have been warned over water quality - including one spot given a Blue Flag last year, environment chiefs have said.
Eight seaside bathing areas failed to hit even basic standards of cleanliness, along with one inland swimming spot on Lough Ennel.
And the Environmental Protection Agency (EPA) warned another 14 bathing areas were not meeting sufficient quality - including Old Head near Louisburgh, Co Mayo, despite securing a coveted place in An Taisce's Blue Flag awards last year.
Environment Minister John Gormley said it was disappointing the areas did not meet the mandatory European regulations.
The EPA also said a small number of bathing spots were consistently poor, with Clifden beach not meeting sufficient standards for the past five years and Balbriggan front strand only once in the last seven years.
It said improving sewage treatment would help but measures to tackle higher summer rainfall were also needed.
Mr Gormley said 500 million euro has been earmarked this year to target environmental issues and public health through improving bathing water standards and supporting the economy.
Seven councils were praised for good water quality at all their bathing spots - Donegal, Dun Laoghaire Rathdown, Kerry, Louth and Meath county councils, and Dublin and Galway city councils.
The 14 beaches not meeting sufficient standards were Loughshinny, Donabate, Rush and Malahide in Dublin; Ardmore, Youghal; Keeldra Lough, Leitrim; Ballymoney north, Wexford; Counsellor's Strand, Dunmore East, Waterford; Coolmaine, Cork; Ballyallia lake, Ennis; Bray beach, Wicklow; Louisburgh; Tra na bhForbacha near Spideal, Galway; and Enniscrone, Sligo.
Dr Micheal Lehane, of EPA Office of Environmental Assessment, said: "Local authorities need to take action to ensure that bathing waters failing to comply with the EU minimum standards are improved. Adequate measures - including the provision of appropriate waste water treatment facilities - are required. It is also important that local authorities effectively inform the general public when pollution events occur, or are predicted."