It says annual investment in infrastructure has dropped by 40%
A new report has found that raw sewage is still being discharged into waterways in 43 areas.
The Environmental Protection Agency (EPA) has found that 142 large towns and cities complied with the mandatory EU waste water treatment standards in 2015, while 29 failed.
It says raw sewage is still discharging from 43 areas - and the planned delivery of treatment plants at half of these areas has now been delayed, by an average of almost two years.
New treatment plants have been built at Ardmore, Dunmore East and Ballylongford to eliminate raw sewage discharges.
It says the discharge of raw sewage from these areas ceased in 2015 - and they are now served by modern plants.
While reported annual investment in infrastructure since 2014 has dropped by 40% from the average levels during the previous decade.
The report claims that a substantial and sustained increase in investment in public waste water treatment infrastructure is needed to eliminate discharges of raw sewage, comply with European Union treatment standards and protect our environment and public health from the effects of waste water discharges.
It says Ireland is not spending enough, and not spending quickly enough, to deal with the problem.
Gerard O'Leary, the director of the EPA's Office of Environmental Enforcement, said: "The pace of resolving waste water treatment needs to improve.
"It is not acceptable that the timeframe to eliminate the discharge of raw sewage from over 20 areas has slipped by almost two years.
"We need to see increased capital investment and improved efficiencies in the delivery of the outstanding infrastructure necessary to protect our rivers, lakes and coastal waters and for a more sustainable quality of life."
The EPA is calling for resources to be targeted at resolving these issues.
It says 13 collection networks require significant upgrades to meet EU standards, and prevent the loss of waste water into the environment.
EPA audits also identified 29 sites with no operation and maintenance programme, which the agency says is a key tool to ensure the correct operation of treatment infrastructure.
Waste water discharges contributed to poor quality water at six popular beaches - including Merrion Strand, Youghal Front Strand and Duncannon.
And waste water from 45 areas was linked with river pollution, which is down from 49 in 2013.