Irish Water is warning that supplies could be restricted until the autumn.
The hosepipe ban will be extended this week to Waterford, Laois, Kilkenny, Limerick, Kerry, and Galway.
It started in the greater Dublin area yesterday to try and counteract falling reservoir levels during the heatwave.
Managing Director of Irish Water, Jerry Grant, does not expect things to improve any time soon.
He said: “My expectation now is that this likely to go on right into September because the kind of drought we are seeing at the moment shows a serious deficit in groundwater levels.
“That means obviously that, unless we get torrential rainfalls in August, which we sometimes do, we will see a very slow recovery, especially in groundwater sources where recharge takes a lot of time.”