Irish Water has said it would be "very optimistic" to think a major boil water notice could be lifted at some point today.
The utility said test results are due this morning on water in affected parts of Dublin, Kildare and Meath.
The notice has left 600,000 people unable to drink their tap water.
An inspection of the Kildare water treatment plant at the centre of the notice is to be carried out this afternoon.
Yvonne Harris from Irish Water said lifting the notice at some point today is unlikely.
Ms Harris said: "Given that we do have to go through the audit this afternoon, we won't have the results until Saturday and it's really up to the EPA as to when they get those results with us.
"So I think today would probably be very optimistic, we are still very, very much hoping that it will be lifted before the long weekend but, as the Taoiseach said n the Dáil yesterday, we cannot guarantee that either."