Inspectors will examine Irish Water’s Leixlip treatment plant today after a fault cut off the supply of clean water to 600,000 people.
A boil water notice remains in place for parts of Dublin, Kildare and Meath.
The notice took effect on Tuesday evening and Irish Water is hopeful it can be lifted by the Bank Holiday weekend.
The HSE has warned that the untreated water may contain parasites that cause diarrhoea and stomach cramps.
Anyone who develops symptoms is urged to contact their GP.
Boil water notice
Irish Water spokesperson Yvonne Harris said it would be "very optimistic" to expect the boil water notice to be lifted today.
She said the inspection at the Leixlip plant will be carried out by the Environmental Protection Agency (EPA) supported by the HSE and Fingal county council.
“That will be one of the most important activities that will drive towards lifting the boil water notice,” she said.
“There are other activities including continuous testing of water and sending those through the laboratory.”
She said the utility expects further test results this morning.
“We are still very much hoping that we will have it lifted before the long weekend but, as the Taoiseach said in the Dáil yesterday, we can’t guarantee that either.is.
Irish Water said the fault at the Leixlip plant “compromised the disinfection process” and sent untreated water into the system.
It has since been fixed; however, the system must be flushed clean before it is passed as safe to drink.
A detailed map of affected areas is available here.