People in the Greater Dublin region use about 148 litres of drinking water every day and risk running out, council chiefs warned today.
The area's largest raw water supply, the Poulaphouca reservoir in Wicklow, has only 154 days` supply at present compared to the normal 200 days.
John Tierney, Dublin City Council manager, has warned the region - which takes in Fingal, Dun Laoghaire-Rathdown, Wicklow, Kildare and the city - needs a new water source.
"Dubliners should use water sparingly and wisely right throughout the year and not just during the dry summer weather, because it is a limited resource and a very valuable one," he said.
"There is a limit to the amount of raw water that can be treated at Dublin's four water treatment plants each day and Dublin's water supply operates to very fine margins, as demand often matches or exceeds supply."
Council chiefs fear that a long, hot summer or a repeat of last January's big freeze, when thousands of homeowners left taps running to avoid burst pipes, would force daily restrictions.
At the peak of the winter, demand soared to an unprecedented 634 million litres a day.
Last month after a long spell of warm weather supplies were running low in Fingal while many counties in the west of the country imposed restrictions, including the Aran Islands of Inis Mor and Inis Oirr, which faced near-drought levels.
The 1.4 million people living in the region drink only about three or four litres a day but Dublin council bosses revealed about 150 litres per person were being used every 24 hours.
Mr Tierney said the amount of raw water available for storage was falling but had not yet dipped to levels that would force restrictions.
But officials warned about 540 million litres of drinking water are needed in Dublin every day and the city's four treatment plants were designed to supply only 518 million litres a day.
The council said the amount of clean water has been very finely balanced between supply and demand for many years and it will continue until key infrastructural projects, including a new water source, are in place.
Dublin City Council has 2,700km of water mains with 1,300km more than 50 years old and the rest more than 75 years old.
Over the last three years, 50km of the 1,300km of water pipes in need of rehabilitation have been replaced in the city.
"The capacity of the rivers in the Dublin region to supply water is limited and we need a new source because demand for water is increasing and will continue to increase," Mr Tierney said.
"Demand is expected to increase to 800 million litres a day by 2031 and we are planning now to meet the expected demand. We are proud of our record of meeting the increasing demand for water over the last decade, with no new water source or increased water treatment capacity.
"It was achieved, among other things, through careful and innovative management by water services staff and they deserve our congratulations."