Households across Fingal served with a second boil water notice in as many weeks could be facing disruption for at least the next 12 months.
Irish Water admitted on Tuesday that the Leixlip plant, which was built 40 years ago, needs major refurbishment works which will take more than a year.
The Environmental Protection Agency warned that the upgrade would not be enough to fix the underlying problem describing the facility as “unfit for purpose”.
On Monday 615,539 residents in north Dublin were issued with a second boil water notice in the space of ten days after heavy rainfall resulted in contamination.
Households in Ashtown, Balbriggan, Baldoyle, Clonee, Clonsilla, Corduff, Donabate, Howth, Lusk, Malahide, Naul, Rush, Skerries, Sutton and Swords were affected.
Senator James Reilly (FG) called for new laws to be enacted so Irish Water could be held accountable for “dereliction of duty”.
Senator Reilly also called for greater powers of enforcement for the EPA following the latest disruption.
He told Dublin Gazette: “I am calling on the Minister to bring forward legislation to give the people of Fingal greater confidence in their water supply and to show that consequences will follow for dereliction of duty.
“It is deeply concerning that yet another boil water notice has been issued in such a short space of time.”
It emerged in recent days that Irish Water failed to act on recommendations made by the EPA earlier this year in a report on the Leixlip facility.
Managing Director of Irish Water Niall Gleeson admitted the plant had repeatedly caused problems but said it could not be taken out of service.
He added: “Ideally we would shut the plant down for six to 12 months and do all these refurbishment works, but because it’s such a critical part of the Dublin water supply, we have to keep it in operation.”
EPA head Dr Tom Ryan said the facility was “not sufficient to manage risk” and warned that the upgrade works would not solve the problem.
He told an Oireachtas Committee: “The level of treatment at this plant is not sufficient to manage the risk posed by the River Liffey source water.
“In particular, there is currently a deficit for treatment of parasites such as Cryptosporidium and Giardia.
“Filter upgrade works are underway to reduce this deficit but will not be sufficient to confirm adequate treatment.”
Fianna Fail water spokesman Darragh O’Brien said the two boil water notices in as many weeks had “rocked people’s confidence in the water supply”.
Cllr Duncan Smith (Lab) said an urgent investigation was needed describing a second major disruption in less than a fortnight as “incredible”.
He told Dublin Gazette: “I can’t believe that we are back in this situation. We need answers from Irish Water whether this is to become a regular occurrence during bad weather this winter?”
Fingal County Council advised households with any questions to contact Irish Water directly at www.water.ie or 1850 278278.