A number of roads and bridges have been 'washed away' on the Inishowen Peninsula in Co Donegal as a result of heavy rain and floods overnight.
One of the worst affected routes is the Muff to Greencastle Road, which usually sees large volumes of traffic.
AA Roadwatch reports that the Mountain Road between Carndonagh and North Pole is among the other roads closed as a result of bridge collapses.
Grainne's Gap, meanwhile, is closed following mudslides.
The majority of the water has now subsided, and the local council is expected to begin assessing the damage to the roads and bridges.
Irish Water reported a number of burst mains following the flooding, while the ESB said lightning strikes and flooding caused power outages for some customers.
Both utility companies were carrying out repair works to restore service to those still affected.
As of midday today, 1,600 customers were still without power.
An ESB spokesperson said: "In some areas that continue to be impacted by flooding, crews may not be in a position to restore power on safety grounds. This is primarily due to the fact that ESB Networks equipment located in flooded areas cannot be safely switched back on until flooding subsides.
"Given the extent of the damage on the Inishowen peninsula, ESB Networks expects some additional outages to be reported throughout the day."
Donegal County Council said it had cordoned off the Swan Park area of Buncrana due to flooding:
— DonegalCountyCouncil (@donegalcouncil) August 23, 2017
Some roads in Sligo and Cavan were also closed or impassable this morning following the heavy rain.
'Relentless and torrential rain'
Inishowen Fine Gael Councillor Bernard McGuinness said he had never seen "anything like it in all my life", noting that the rain was "relentless and torrential".
He observed: “Roads which never flood did this time. It was a weather event not seen here in living memory with 70mm of rain in just a few hours.
“Damage in many areas is severe. I want to pay tribute to local people, emergency services and council staff who worked through the night."
Donegal TD and the Government's Chief Whip Joe McHugh said the flooding in his county will be treated as a "priority case".
“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," he added.
Junior Minister For Flood Relief, Kevin 'Boxer' Moran, will visit the affected area later today.
Fianna Fáil's Robert Troy, meanwhile, called for emergency funding to be made available to repair damaged transport infrastructure.
He argued: “It’s clear that emergency funding will be needed to help repair the damage to roads and bridges in particular. Minister [Shane] Ross needs to put in place a contingency fund to help repair the damage and ensure that works are carried out without delay.
"This is an exceptional situation which requires an exceptional response."
The PSNI also warned of hazardous conditions on the roads this morning in parts of the North:
Drivers are warned of hazardous conditions due to floods especially in Tyrone & Derry/Londonderry. Please slow down & drive with care.
— PSNI (@PoliceServiceNI) August 23, 2017
Emergency services in Northern Ireland reported an "extremely busy night" as a result of the weather conditions.
In a Facebook post, Belfast Coastguard said: "At one point three Coastguard Rescue Helicopters were tasked to [Northern Ireland]. Rescues ranged from people trapped in vehicles, houses and cut off by flood water.
"Another fine example of inter-agency working, alongside our colleagues from Northern Ireland Fire & Rescue Service, Northern Ireland Ambulance Service and Police Service of Northern Ireland."
Additional reporting by Nicole Gernon