The Environmental Protection Agency is to carry out an audit tomorrow of the Water treatment plant at the centre of a boil water notice affecting more than 615,000 people.
The HSE will also attend the audit at the Leixlip plant.
A spokesperson for the EPA said this audit, the second in two weeks, does not automatically mean the boil water notice will be lifted tomorrow.
But it is hoped the audit will provide more information as to when normal service can resume.
Earlier, Irish Water said changes will have to be made as to how the Leixlip plant operates before the boil water notice is lifted.
An audit by the EPA on 24 October found a number of issues at an older part of the plant and the EPA said some of these issues had previously been flagged.
It said it was unacceptable that Irish Water and Fingal County Council had not acted on those issues.
Irish Water said changes to how the plant operates will have to be made before the boil water notice is lifted.
Yesterday, it said might be lifted by tomorrow now it is saying it does not know when people in the areas affected will be able to drink their tap water.
It says it is still trying to agree what criteria will apply before it can resume normal service.
The issues are in the older treatment plant at Leixlip where upgrading and remedial work has begun.
The EPA did an audit of the plant following the boil water notice last month but it now seems it may have to do another and it is not yet clear how long that will take.