Raw sewage is still being pumped into Ireland's waterways, with 29 larges towns and cities around the country failing to meet EU standards.
A new EPA environmental report claims "significant funding" is needed to treat waste water if the country is to avoid pollution and health risks.
The building of treatment plants in many areas have been delayed by up to two years.
The report suggests the problems are due to a lack of investment in infrastructure which has dropped by 40% in the last decade.
Gerard O’Leary, 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."
David Flynn, programme manager of the EPA’s Office of Environmental Enforcement said: “EPA analysis has found that better operational practices should resolve issues at one fifth of poorly-performing plants.
“Irish Water needs to improve the performance of these plants to get the most from the existing infrastructure.”