The Leixlip Water Treatment Plant is set to be audited tomorrow as a boil notice remains in place.
Over 615,000 people throughout Dublin, Kildare, and Meath are affected.
They haven't been able to drink their tap water since Monday evening.
The HSE and senior management from Irish Water and Fingal County Council will attend tomorrow's audit of the Leixlip site.
General Manager of Irish Water Eamon Gallen said, "Irish Water understand and appreciate that this Boil Water Notice is having a significant impact on homes and businesses.
We are pleased that we have had the results of two water samples and both of these have been satisfactory.
The EPA and HSE may include these satisfactory samples in their audit, but we will be bound by their requirements in this matter."