Irish Water had been warned about issues with the Leixlip Water Treatment Plant six years ago.
Boil water notices were imposed on 600,000 people across the greater Dublin area twice last year.
They were the largest boil water notices in the history of the state.
The Irish Independent reports that failing filters were reported in 2014, and it was also said that they may not be able to catch dangerous bacteria.
Irish Water has said that supply is not guaranteed, and while repair works are carried out at the site more boil water notices may be necessary.
The utility has not said what the full extent of the works that are required on the plant - or how much it will cost.
Irish Water, as well as representatives from the EPA, appeared before an Oireachtas committee in November, which was told the watchdog had been warning Irish Water about issues at the plant which led to the boil water notice being issued.