Just half of Ireland's sewage in 2021 was treated to European Union standards set to protect our environment, a new report from the Environmental Protection Agency (EPA) has found.
It said this is "well below" the EU average of 90%, and that poorly treated sewage continues to harm the quality of our rivers, lakes and coastal waters.
The EPA said Irish Water has "no clear plan" to improve treatment at 27 priority areas, where wastewater discharges are impacting on waters.
However, works to eliminate raw sewage flowing into seas and rivers from 32 towns and villages have started, or are due to start, by 2024.
The report said the country's largest treatment plant, at Ringsend in Dublin, is "overloaded and fails to consistently treat sewage to the required standards".
A major upgrade of the plant began in 2018 and is due to be completed in 2025.
The EPA added: "Construction work to provide treatment for the 32 areas discharging raw sewage listed in the report is either ongoing or due to start in the next two years.
"It is essential that Irish Water delivers these projects as soon as possible," it said.
EPA Director Dr Tom Ryan said: "This report shows that targeting investment at the priority areas identified by the EPA is delivering improvements in water quality, and the elimination of raw sewage discharges from Cobh and Castletownbere in Cork are good examples of progress.
"However, it will take a high level of sustained investment over the next two decades to bring all treatment systems across the country up to the required standard to protect the environment and public health."
He added: "Irish Water has failed to produce action plans to improve treatment at one-third of the priority areas identified by EPA where wastewater is harming our rivers, lakes and coastal waters.
"It is essential that Irish Water provides clear, site-specific action plans and time frames to improve treatment in these areas, and it needs to allocate the necessary resources in its next investment cycle to implement and complete them and to resolve the associated environmental harms."