A hosepipe ban comes into effect at midnight tonight as water levels have dropped critically low with little rain in sight.
The ban applies to all non-essential uses of water and will last six weeks, with the hope it can be lifted on July 21.
Using hosepipes for gardens, cleaning and leisure activities is banned for homes and businesses, with a €125 fine applying.
Irish Water said the move was necessary as a hosepipe turned on for just one hour used up as much water as an average family on a whole day.
The company said it was taking the step to ensure there was enough water for drinking, washing and other essential uses; to avoid having to impose restrictions on flow, and to minimise the risk of taps running dry.
"It is essential that our water supply is protected if we are to avoid restrictions and outages over the coming weeks and months," said managing director Niall Gleeson.
Last month was the driest May since 1850, while the Greater Dublin Area had its driest spring on record and also one of its warmest.
Showers over the weekend and the few forecast for this week will go nowhere near to making up for the lack of rain over the last few months.
Irish Water said it would take at least two months' worth of rain to fall in a 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 Mr 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.
He said Irish Water was ramping up repairs to leaks, which also strain 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," Mr Gleeson added.