The contents of toilets, sinks and drains are being dumped directly into rivers, loughs and the sea at 45 locations around the country, inspectors have found.
In the worst example sewage and waste water from Carrigaline, Crosshaven and Ringaskiddy - equivalent to 116,000 people - is being pumped untreated into Cork harbour.
The Environmental Protection Agency (EPA) said discharging dirty water was directly responsible for poor quality at seven popular swimming spots - Youghal Front Strand in Co Cork, the south beach in Rush, Co Dublin, Ballyloughane and Clifden in Galway, Ardmore in Waterford, Lough Ennel in Westmeath, and Duncannon, Co Wexford.
It warned the amount of waste water and raw sewage being pumped out from villages, towns and cities was equivalent to 200,000 people each using 140 litres of water a day.
Locations affected include popular tourism, beach and surfing spots like Bundoran, Kinvara and Kilkee.
Noel Byrne, senior inspector with the EPA, said: “Unfortunately we are dealing here with a legacy of underinvestment over decades and that poses a significant challenge for Irish Water. It is long overdue.”
The EPA’s Urban Waste Water Report 2014 ranked Cork, Donegal and Galway as the worst counties for untreated sewage with more than half the discharges recorded in those areas.
Under European rules, by the year 2000 Ireland should have had secondary treatment of sewage in place for towns and cities with populations of more than 15,000.
The EPA noted improvements in waste water treatment and also said a new sewage plant is planned for the Carrigaline, Crosshaven and RIngaskiddy areas by the end of next year
It also said only one river, the Bredagh in Moville, Co Donegal, was classed as seriously polluted compared with six rivers in 2013.
The EPA survey also found operational improvements were needed at 57 urban areas where there was sufficient treatment capacity but effluent quality standards were failing.
It also found a fifth of pollution incidents at sewage plants could be prevented by better management and operation.
After independent monitoring at 263 treatment plants, 71 were found to have failed to comply with effluent quality standards stipulated in licences.
The 45 locations where raw sewage was being pumped out to sea were:
:: Co Clare - Ballyvaughan, Clarecastle, Kilkee, Kilrush, Liscannor and Ballycotton
:: Co Cork - Castletownbere, Castletownshend, Cobh, Inchigeelagh, Passage West/Monkstown, Ringaskiddy/Crosshaven/Carrigaline, Ringaskiddy village, Timoleague, Whitegate/Aghada and Youghal
:: Donegal - Bundoran, Burtonport, Coolatee housing scheme, Falcarragh, Kerrykeel, Kilcar, Killybegs, Moville, Ramelton, Rathmullan and St Johnston
:: Galway - Ahascragh, Carraroe, Roundstone, Kinvara and Spiddal
:: Wexford - Arthurstown, Ballyhack, Duncannon and Kilmore Quay
:: Waterford - Ardmore and Dunmore East
:: Wicklow - Arklow and Avoca
:: Rush in north Dublin, Ballylongford in Co Kerry, Omeath in Co Louth and Belmullet and Killala in Co Mayo