Pumps supplying a water tower in Blanchardstown were shut down overnight.
Irish Water says it looks like a lightning strike caused of a major fault.
Water supplies to parts of Blanchardstown, Mulhuddart, and rural parts of Fingal north of Dublin 15 were affected.
"We would like to apologise for any inconvenience caused and assure customers that we are doing all that we can to return service as quickly as possible," it added.," Irish Water said in a statement. "The outage appears to be caused by a lightning strike to a pumping station."
Workers managed to restart the pumps by lunchtime.
Fingal County Council says it may take several hours to get water pipes in the area back up to full pressure -- especially those supplying homes and businesses on higher ground.