Update 2pm: A major clear-up operation is under way in the north-west of Ireland and the North after heavy rain left a trail of destruction.
Bridges collapsed, roads gave way and cars were washed away in overflowing rivers during a night when 63% of August's average rainfall fell within nine hours.
More than 100 people had to be rescued from vehicles caught in flash floods. Firefighters saved 93, with the Coastguard helping dozens of others.
Police, ambulance crews and search and rescue teams were also involved in the response.
Temporary accommodation centres have been opened to house those whose homes were flooded.
Counties Donegal, Derry and Tyrone were worst hit.
There were some dramatic images of devastation on Wednesday as the rescue operation turned to clean-up and repairs.
Downpours swept in from the Atlantic Ocean, hitting Co Donegal on Tuesday afternoon before moving across the North.
Firefighters responded to 85 separate flooding incidents on Tuesday night and into the early hours of Wednesday.
Alan Walmsley, Assistant Chief Fire Officer at the Northern Ireland Fire and Rescue Service, said: "Firefighters rescued 93 people who were trapped by flood water either from their cars or from properties.
"Our regional control centre (RCC) received 402 emergency 999 calls between 7pm last night and 4am this morning.
"At its peak time, between 9pm and 11pm, RCC handled an emergency call every 45 seconds.
"Firefighters worked in extremely challenging circumstances to reach people.
"A number of roads were impassable due to flooding and crews had to take alternative routes to attend some incidents.
"We worked closely with colleagues from the Irish Coastguard Agency, PSNI (Police Service of Northern Ireland), ambulance service, Foyle Search and Rescue and local councils to respond to incidents to assist members of the public.
"Our specialist teams, including the water rescue teams and specialist rescue team from Belfast, were deployed and worked alongside local operational crews.
"This was a very busy and challenging night for fire crews and our regional control centre personnel.
"I commend them for their professionalism in responding to such challenging circumstances as efficiently as they did throughout the night."
Donegal TD Joe McHugh said the response to the Donegal flooding would be a priority.
Mr McHugh said: "My first thoughts this morning are with the families affected by this severe weather event.
"I also want to thank the emergency services, Donegal County Council and the incredible volunteers who worked through the night to save property and to save lives.
"Inishowen has been badly affected with some roads and bridges washed away.
"Many roads across the east of the county are also affected.
"It is important now that an initial assessment of the damage caused is carried out and I've been in contact overnight with a number of officials including the Office of Public Works."
Update 10.10am: Some 3,500 properties are without electricity in Donegal this morning due to lightning strikes and flooding.
The ESB said the poor weather conditions knocked out supply to 25,000 customers in the northwest, midlands and Dublin regions last night. Crews are now working to restore supply.
Anyone who finds fallen cables is urged to contact the emergency number 1850 372 999 immidately.
Update 8.45am: A number of roads and bridges have been washed away on the Inishowen Peninsula in Co Donegal after heavy rain and floods overnight.
One of the worst affected routes is the Muff to Greencastle Road, which usually sees large volumes of traffic.
Gardaí are advising people not to make unnecessary journeys and that, if travelling on back roads, to slow down and expect delays.
The majority of the water has now subsided and the council is expected to begin assessing the damage to the roads and bridges.
Efforts are underway to restore power to hundreds of homes around the country.
The ESB says lightning strikes knocked out supply to around 2,000 properties last night.
Services in Roscommon, Dublin, Limerick, Louth, Wexford and Donegal are affected.
Many homes were restored to power last night, but hundreds are still without power this morning, as floods and heavy rain hit the north-west and west.
A nationwide status orange thunder warning was issued by Met Éireann yesterday, on top of a separate status yellow rainfall warning for Connacht, Cavan, Monaghan and Donegal.
The weather forced some homeowners to stay in their homes, while others were trapped in their cars by rising waters.
The outlook for today is better, though some showers are forecast. Met Éireann says it will be mainly dry and bright this morning but with cloud gradually building up towards midday.
Showers will be well scattered at first but they will be more frequent during the afternoon and evening, with some heavy. Top temperatures will be 18C.
The weather will remain mixed tomorrow and Friday, but will turn drier over the weekend, with just an occasional shower before rain returns to the west on Sunday evening.