Three major treatment plants which provide water to more than half a million people need to be modernised to secure supplies, environmental experts have warned.
An unstable 150-year-old tunnel from the Vartry reservoir in the Wicklow mountains should be replaced while the Lee Road facility, on a flood plain in Cork, must be moved to higher ground and the Staleen plant in Co Meath needs to be overhauled.
The Environmental Protection Agency (EPA) named the three sites on a Remedial Action List which plans works for a total of 240 facilities.
Dara Lynott, EPA deputy director general, said more than 80% of works targeted in 2008 will be completed this year.
He said: "We are keen to draw attention to it because there`s an obvious risk using a tunnel that is 150-years-old. It's outside what we would normally look at but we included it because we are looking at quality and we are looking at the seriousness of the situation."
The EPA said significant investment will be required to complete upgrade works on the three plants supplying a population of 511,135, including up to 30 million euro for the new Callow Hill tunnel at Vartry.
Dublin City Council claimed almost a year ago that the Vartry line, a rock-lined passage in the mountains near Roundwood, was unstable and could collapse at any time. The 4km-long tunnel, constructed in the 1860s, brings drinking water to 335,135 people but work is not likely to take place until the end of 2015 at the earliest. It carries 80 million litres to Greater Dublin, about 20% of the total.
Elsewhere, the Lee Road plant, supplying 123,000 people in Cork city, must be moved to higher ground after it was inundated by floods in 2009. Council chiefs are aiming for the move to be completed by the end of 2014.
Staleen, supplying 55,000 people in Drogheda and counties Meath and Louth, is failing a test for aluminium standards which suggests it may also be failing other quality standards. Work will not be done until September 2012.
The EPA drinking water quality report also showed Ireland is now down to the benchmark seen in England and Wales and the Netherlands for preventing E.coli getting into supplies.