More than 50 homes and many businesses were flooded and several thousand left without power after 50mm of rain fell in a seven-hour period across Cork.
Council bosses said pre-emptive action helped reduce the impact of the overnight flash floods which left more than a dozen areas under water.
Emergency services in the city received 128 calls for help following floods, including 45 callers needing direct rapid response.
Met Eireann said the rain, including thundery downpours and hail, is expected to move north across the country today after torrential showers dumped up to 70mm locally in some parts of Cork.
Forecasters said heavy rain fell between 8pm and 3am on top of already saturated ground, compounding the impact of torrential showers.
The deluge occurred after several hours of rain across Belfast and parts of counties Down and Antrim left 1,000 homes without power and 700 people needing the help of emergency services.
Cork City Council said 20 homes were hit by the floods in Park Court, Ballyvolane, and another eight in the Commons Road area.
Also in the city the authority said a significant number of properties, business and residential, in Blackpool Village and Watercourse Road were inundated in the early hours of the morning.
Elsewhere, the council said localised flooding was reported on the Old Kinsale Road near the business park, Sarsfield Road roundabout, Monaghan Road, Centre Park Road and Turner’s Cross.
“The council’s advance work in gully cleaning throughout the city assisted in preventing more localised incidents of flooding,” officials said.
Across the county Douglas and Clonakilty were worst hit.
Up to a metre of water was reported on the main streets of Douglas village with 3,000 homes without electricity, while officials warned there was no access in or out of Clonakilty.
Glanmire on the edge of Cork city was also badly hit.
Cork City Council said its flood emergency response plan swung into effect at 3.30am and the crisis management team met at 7am.
It said staff from the fire and emergency services and roads and drainage units followed a swift and co-ordinated response to incidents throughout the morning.
“All council services associated with a flood response continue to be activated and will continue throughout the day to respond where required,” a spokesman said.
Weather and river levels will also be monitored through the day.
Cork County Council said it was alerted to a weather warning at 5pm yesterday and dispatched teams to carry out checks at key flooding spots.
“The rainfall has now abated and water levels are dropping,” a spokesman said.
“But roads remain impassable in a number of areas including Clonakilty town, Douglas and Sarsfield’s Road.”
The council chiefs said the region has been hit by three times the average monthly rainfall total for June.