The problem has resulted from very heavy rainfall which has washed large amounts of organic matter into reservoirs, increasing the turbidity – or ‘cloudiness’ – of the source water above acceptable levels.
The older of two water treatment plants at the Leixlip facility has been unable to react fast enough to deal with the rise in turbidity.
However, that older plant produces 20% of the drinking water for the entire greater Dublin area and there is insufficient capacity elsewhere to replace it.
Irish Water said the areas impacted were the same in a previous boil water notice issued last month.
It includes parts of Fingal, areas in Dublin City Council, parts of South Dublin County Council, parts of Kildare and Dunboyne in Meath.