Irish Water has claimed it is hamstrung by a lack of funding while acknowledging improvements to the network around the country are behind schedule.
The body was responding to criticism levelled at it by the Water Advisory Board (WAB), which said 19 large towns and cities do not meet EU standards set to protect the environment.
WAB, an independent statutory body established in 2018, said that in addition to the 19 towns and cities cited, a further 33 towns and villages will continue discharging raw sewage after 2021 because they will still not have a wastewater treatment plant.
Responding to the findings, Irish Water said it "acknowledges the size and scale of the legacy issues, planning and land acquisition delays, and the need to reprioritise investment to respond to emerging needs".
Funding constraints have resulted in slower progress than initially forecast, Irish Water added, a factor WAB said was significant in Irish Water's ability to carry out its work.
Irish Water said it has invested €250m to stop the discharge of almost half of all untreated sewage in Ireland, with 15 new wastewater plants delivered to date.
It plans to begin the construction of 12 new plants next year and to address the remaining sites throughout 2022 and 2023, subject to planning and funding.
"We are committed to working with local communities to deliver these critical projects in areas that have never previously been served by wastewater networks or plants," a spokesperson said.
According to Irish Water, the list of water and wastewater projects and programmes is "continuously being refined" subject to budget, technical and environmental constraints, as well as statutory approvals.
Scottish Water International was commissioned by Irish Water to review its internal processes, and implementation of their recommendations will play a role in the future delivery of capital works, Irish Water said.
Irish Water managing director Niall Gleeson, said: “Despite a range of challenges to the delivery of wastewater infrastructure projects, including the poor condition of the legacy infrastructure and planning and land acquisition delays, Irish Water is making significant progress.
"Over the last six years, Irish Water has targeted the resources available to areas with the greatest population or volume of discharge. As a result, we have succeeded in reducing the amount of raw sewage discharged by 50% to date."
The completion of the Cork Lower Harbour Drainage Project in 2021 is on track and the construction of the Arklow Wastewater Treatment Plant, which is due to begin in 2021 subject to funding, will address the majority of the remaining untreated discharge, he added.
"Next year, with funding in place we will start construction in at least 12 other locations across the country, with the remaining projects due to start in 2022 and 2023.”