Almost half of waste water treatment plants in Ireland do not meet national or EU standards.
A new report by the Environmental Protection Agency also found that mandatory water treatment does not take place in 11 large urban areas throughout the country.
The agency said the treatment is ten years overdue in Bray and Ringaskiddy.
The situation is having an impact on bathing water in Clifden, Co Galway. Discharges are causing serious pollution to the River Bredagh in Moville, Co Donegal.
There are also eight urban areas - including Dublin, Cork and Kilkenny - that have not met the requirement to reduce nutrients such as phosphorous and nitrogen.
The EPA said the level of performance is poor and needs to improve and further investment is required to change the way these plants are operated and maintained.
The findings highlight the challenges facing the Government, which plans to create a body to take over the provision of water from local authorities as part of its proposals for water charges.
