The level of treatment at the Leixlip Water Treatment Plant is not sufficient to manage risk posed by the River Liffey source water, according to an audit by the Environmental Protection Agency.
The examination took place after a boil notice was issued by Irish Water on Tuesday of last week affecting more than 600,000 people in Dublin, Kildare and Meath.
It followed issues with the treatment process at Leixlip, which may have compromised the disinfection process making the water unsafe to drink.
The notice was lifted last Friday.
The audit found there was a blockage at the treatment plant, which was made up of small black particles or debris originating from two storage tanks that are approximately 25 years old.
Deterioration of the tanks' internal lining resulted in flakes being drawn into the pumps and then into the dosing line, which caused the blockage.
The EPA described the failure by Irish Water and Fingal County Council to implement the recommendations of a previous audit conducted at the Leixlip plant last March as "unacceptable".
The agency has suggested that its findings and recommendations be addressed at all other treatment plants operated and managed by Irish Water. Irish Water has one month to respond.
The company said the recent events "fell well below our standards".
Responding to the findings, Irish Water has said it was committed to the delivery of a safe and secure supply of water to all its customers.
Irish Water and Fingal County Council are currently reviewing the findings of the audit, following which they will outline the plan for the implementation of the EPA's recommendation/.
That will include details on the actions already taken and those in progress.
Irish Water said it acknowledged and understood the impact of the boil water notice and expressed regret over the inconvenience caused to homes and businesses.
It said: "We are committed to working to minimise the risk of any future incident."