Pubs, hotels and homes face having no water at night at the height of tourist season as Irish Water introduces restrictions across Dublin and Wicklow from Monday.
Thousands of households and businesses will be affected from 10pm to 5am for at least a week, and the utility said it can't guarantee supplies won't be off.
More than 30 locations, from Bray in Wicklow to Finglas on Dublin's northside and Lucan to the west, will be hit.
While most customers should have water from kitchen taps, those in multi-storey buildings, on higher ground or at the edge of the network could see it reduced to a trickle.
Irish Water said the move was needed to "protect future water supply and avoid widespread outages in the autumn".
"What we're trying to do is maximise the amount of water we can save while minimising the impact on homes and business. It's very hard to do," corporate affairs manager Kate Gannon said.
"For general customers, some on high ground and remote ends of the network will experience low pressure but shouldn't experience a loss of supply. For non-domestic customers, the practice is to have 24 hours storage on site.
"If you're in an older building, it may be harder. We can't promise everyone will feel nothing."
Areas around major hospitals will not be affected, with the utility saying the need for restrictions was balanced with how much water could be saved.
While the hosepipe ban and conservation efforts by the public had saved more than 40 million litres a day, it was not clear how much additional water would be saved by the restrictions.
"We have to take this extra step to try to catch up on the demand over the last weeks," Ms Gannon added. "We're hoping the changes people have made are ones they can sustain. We don't expect a massive reduction, but we hope to gauge how much we can get back."
More than 25 supplies across the country already have night restrictions, including Athlone and parts of Galway City.
However, Irish Water warned that unless there was sustained rainfall, additional measures could be imposed, including day-time restrictions.
It has given schemes a 0 to 5 risk rating, with levels 0 to 3 seeing no day-time restrictions.
Level 2 restrictions mean a reduction in pressure at night to allow water to reach the ground floor of a two-storey house.
Level 3 restrictions would be from 10pm-5am, extended to 9pm-7am as required.
Levels 4 and 5 would impact on supply during the day, with rolling shut-offs from 10am-4pm and 9pm-7am.
People with supply issues should contact Irish Water on 1850 278 278.