A hosepipe ban comes into effect across the country at midnight tonight as water levels drop critically low with little rain in sight.
The ban will last six weeks with hopes that it can be lifted on July 21 – but only if the current drought ends.
All use of hosepipes, for gardening, cleaning and leisure activities, is prohibited for households and businesses.
Irish Water said the move was necessary as a hosepipe used as much water in one hour as a whole family in a day.
The utility said it was taking the step to safeguard water for essential purposes, to avoid having to impose restrictions on flow and to minimise the risk of taps running dry.
Managing director Niall Gleeson said: “It is essential the water supply is protected if we are to avoid restrictions and outages over the coming weeks and months.”
Rivers, lakes, springs and groundwater sources that feed the reservoirs supplying hundreds of thousands of people are already in drought and others are on the verge of drought after an almost complete absence of rain since just before the lockdown.
Last month was the driest May since 1850 while the Greater Dublin Area, Westmeath, Sligo and Tipperary have all had their driest spring on record.
But the problem is countrywide, with rainfall totals in every county below average for the season while temperatures have been above average.
The weather forecast is for a continuation of drier than normal conditions which will further exacerbate the situation.
Showers over the weekend and the few forecast for this week will go nowhere near to making up for the prolonged lack of rain over the last few months.
Irish Water said it would take at least 100mm of rain -the equivalent of two months' worth - to fall in the next few weeks, followed by normal summer rainfall levels after that, before the threat to supplies would pass. Since lockdown the average person has used an extra 24 litres of water a day, partly due to greater handwashing and cleaning, but Niall Gleeson said warm weather activities had added to demand.
"Such weather brings people into their gardens and makes the use of hoses more likely. Similarly with children confined to home, it can be tempting to use paddling pools," he said.
"However, using a hosepipe for one hour is the equivalent of the daily water usage of an average family and this is evidently a non-essential use of water."
As businesses reopen in large numbers this week, the additional surge in activity is expected to strain supplies further.
The ban, officially called a water conservation order, is legally binding and comes with the threat of a €125 fine.
However, the last time orders were imposed, during the 2018 summer heatwave, Irish Water adopted a policy of persuading people in breach to stop flouting the regulations and no one was fined. The same policy is being adopted initially this time too, although Irish Water have made it clear the situation is more critical now than in 2018.
Water levels in many areas are already at 2018 levels, a month earlier. In 2018, the ban was issued only for the Greater Dublin Area initially before being extended nationwide.
Supplies in some areas are more stressed than others but the knock-on effect is widespread as water may have to be shared to ensure no area goes without.
The problem has been building since early March and already parts of Galway, Donegal and the midlands have had night-time restrictions.
High demand is not the only challenge to water supplies - 45pc of all treated water is lost through leaks before it ever reaches the taps.
Mr Gleeson said Irish Water was ramping up leak detection and repair but he appealed to the public to help with conserving supplies.
"The key messages are to leave the hose and the pressure washer in the shed, don't use paddling pools, reuse household water for the garden, and take shorter showers. We are calling on everyone to play their part," he said.