Irish Water is warning that supplies in the Greater Dublin Region could fail after 70 days.
Current levels are still under threat following the heatwave.
The hosepipe ban remains in place around the country with restrictions in place in some areas.
Head of Irish Water Jerry Grant says further conservation is needed.
"About the 10th or 12th of May, we had 150 days storage in Poulaphouca [Reservoir]. It's an enormous resource of water and it saves the city year in, year out," he said.
"We used 80 days of that storage in 83 days. In other words, almost nothing was contributed to the storage in that period.
"We've about 70 days of usable water left for the Greater Dublin Area, 1.6m people. We could be 90 days before that lake starts to rise depending on how dry the autumn is."