People are being urged to conserve water as the heatwave continues.
Uisce Éireann said water is being consumed more quicky that it can be produced and replenished in local reservoirs.
A Status Yellow high temperature warning for 13 counties will be in place until 7am tomorrow.
The warning covers Carlow, Kildare, Kilkenny, Laois, Longford, Meath, Offaly, Westmeath, Wexford, Cork, Limerick, Tipperary and Waterford, with temperatures expected to reach as high as 30C in places.
A Met Éireann weather advisory said the prolonged spell of hot weather will last into next week.
A separate Status Yellow high temperature warning has been issued for Clare, Cork, Kerry, Limerick, Tipperary, Galway, Mayo, Kilkenny, Laois, Offaly from 1pm tomorrow until 7am on Monday.
Met Éireann said highest daytime temperatures could reach higher than 27C, while at night temperatures could stay as high as 15C.
The forecaster said heat stress and uncomfortable sleeping conditions can be expected. It also warned of water safety issues as people flock to lakes and beaches, as well as the possibility of forest fires.
Uisce Éireann has appealed to farms, households and businesses in six areas to conserve water following exceptionally high demand.
These areas are the Cooley Peninsula, west Cork and parts of north Cork, the Greater Dublin area, south Leitrim and Tipperary.
Uisce Éireann said the imposition of night-time restrictions in the Greater Dublin area were being considered and may be introduced if the level of water usage is not reduced.
Yesterday, the utility said the increase in water usage is being driven by a combination of garden watering, power hosing and other outdoor domestic use and that the pattern of continued high use is increasing risk to supplies.
It said reservoir levels are dropping by up to 30 million litres on peak usage days, which it said shows the need for urgent conservation.