Overnight water restrictions will apply to almost every part of Greater Dublin from Monday.
Pressure will be reduced between 10pm and 5am
This is in order to manage demand resulting from the heatwave and aimed at avoiding widespread outages in the autumn.
Meanwhile, there is a warning a proposed pipeline from the Shannon to provide an alternative water supply for Dublin could cost €1bn..
Dublin City Councillor Ciaran Cuffe said even if it goes ahead, there will be ongoing costs.
He said: "You have to keep pushing the water from the Shannon to Dublin, unlike our existing water supply where an awful lot of the water flows pretty much without pumping down from the Dublin and the Wicklow mountains.
"If we're to bring it all the way over from the Shannon we will have to keep the pumps going and there will be an ongoing charge running into the tens of millions."