About 10 tonnes of raw sewage and water from sinks and drains is pumped into rivers and seas around Ireland every minute, environment watchdogs have warned.
review of the standards of waste treatment in towns and cities found 16 plants which were due to be built or upgraded this year have been delayed until 2021.
The Environmental Protection Agency (EPA) said sewage from 120,000 people - on average 120 litres per person - still enters the tides and rivers untreated at 44 areas each day.
The scale of the problem was laid bare yet again months after the European Commission warned it was taking Ireland to court over the practice and leaving the country potentially liable for millions of euro in anti-pollution fines.
That case relates to missed deadlines for sewage plants to be built or upgraded by the end of 2000 and 2005.
But in its latest report on waste water standards for last year the EPA said five towns are hoping to see work on new sewage treatment plants finish this year - Youghal, Belmullet, Rush, Bundoran, and Killybegs.
But David Shanon, an EPA inspector, said: "There's not enough money being invested in waste water treatment.
"We are spending 100 million euro less per annum than we are a decade ago.
"We have this legacy of under-investment. We need to see more money going in."
The EPA said four swimming spots - Loughshinny in north Dublin and Merrion Strand near Ringsend on the south side and Clifden beach and Ballyloughane in Galway - were not safe enough for bathing because of the sewage issue.
The report warned that waste water treatment at 50 of the country's 185 large towns and cities misses pollution and public health standards.
Gerard O'Leary, director of the EPA's Office of Environmental Enforcement said: "Wastewater from over half our population failed to meet environmental standards.
"For many years Ireland failed to address the deficiencies in wastewater treatment. Substantial and sustained investment is now required to protect our valuable waterways and protect public health."
The EPA also warned about Ringsend, where a relatively new treatment facility is operational.
But the facility is essentially overloaded as it was designed for a population of 1.6 million but handles sewage from two million and also does not have specialist treatment to remove specific nutrients from water being pumped into Dublin Bay.
Irish Water's head of asset management Sean Laffey said: "The EPA report correctly reflects the size and scale of the challenge facing Irish Water as we work to meet the needs of homes and businesses around the country and facilitate future growth."
It said it was spending about 326 million euro on the issue up to 2021.