An inspection of the Kildare water treatment plant at the centre of the boil water notice will take place later.
600,000 people in parts of Dublin, Kildare and Meath remain under the warning, which took effect on Tuesday evening.
It's after a mechanical fault at Irish Water's Leixlip plant allowed untreated water to enter the system.
The utility says it hopes to have the notice lifted for the majority of those affected before the long weekend.
Irish Water says it would be "very optimistic" to think a major boil water notice could be lifted at some point today.
People aren't able to drink their tap water and it's reported that some stores in affected areas are rationing bottled water.