A boil water notice has been issued for a second time for people living in parts of counties Dublin, Kildare and Meath, with over 600,000 people affected.
The issue has arisen as a result of weather affecting water quality at the Leixlip Water Treatment Plant.
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.
The utility said plant operators reacted quickly to the issue before alarms were activated and the plant was shut down.
Irish water said it was faced with a choice of imposing severe restrictions to the water supply for a long period of time or having water for sanitation purposes such as showering and flushing toilets.
It said it was left with no option but to resume production at the older plant in the knowledge that a boil water notice would be necessary.
The notice has been issued with immediate effect for the entire supply.
The action is being taken as a precautionary measure and follows issues with the treatment process at the plant that may have compromised the disinfection process, which makes the water safe to drink.
Irish Water has apologised for the impact and inconvenience caused by the issue, stating that public health is its number one concern.
As a result, those in affected areas must boil water used for drinking, the preparation of salads, brushing teeth and making ice.
Further information can be found on the utility's website, or customers can contact the helpline on 1850 278 278.