Update 6.30pm: Met Éireann has issued a fresh rainfall warning for Donegal this evening.
Around 200 homes were destroyed and roads and bridges were washed away when 80 mm of rain fell in just a few hours on Tuesday night.
The weather forecaster is warning that further spells of heavy rain are expected to affect the northwest of the country this evening until 6pm tomorrow evening.
The warning affects Donegal, Galway, Letrim, Mayo and Sligo.
Meanwhile, the Taoiseaqch Leo Varadkar has said he is shocked at the extensive damage caused by wild weather in Donegal.
In a video message, he sympathised with those affected and told locals in Donegal of the help available to them.
Update 3pm: The Government has said the amount victims of this week's Donegal floods receive will depend on how badly their house is damaged and their ability to meet the costs.
The Humanitarian Assistance Fund provides money for food, clothes and personal goods.
The Department of Social Protection said it would prioritise those most in need and those with losses which are not covered by insurance.
John McLaughlin from Donegal County Council said Council has been dealing "devastating" damage.
He said: "Places that would have been under one foot (of water) in the past would have been under three and four feet (this week). It has been coming down hillsides and crossing roads where you would not normally expect it whatsoever. The level of erosion and ripping apart the edges of roads and bridges - absolute devastation.
"Whole estate are flooded out. People were up to shoulder height in water within minutes; oil tanks and fences were washed away."
Update: 11.39am The Department of Social Protection has activated its humanitarian assistance fund for Donegal residents.
It follows devastating flooding earlier this week which has destroyed bridges and washed away roads.
However, the Department has not released details of how much money has been made available.
Residents have been questioning whether Donegal County Council was sufficiently prepared for the disaster.
Joe McHugh, the Government Chief Whip and Minister of State for Gaeilge, Gaeltacht and the Islands, visited some of those affected in Buncrana, Bridgend and Burnfoot in Donegal, and described scenes of devastation.
"It is essential that an emergency aid package is put together so we can help families rebuild their lives and so that bridges and roads washed away in the floods are restored as soon as possible," the Donegal TD said.
"We have heard incredible stories of neighbours saving neighbours at the height of the floods. It is a miracle no lives were lost during this weather event."
Six major bridges and roads will need to be repaired, including on the busy Moville to Muff road on Inishowen.
Mr McHugh, who is visiting more affected communities, said he would work with cabinet colleagues over the next few days to arrange a special emergency funding package for those hit by the floods.
Cars were washed away in places when the violent thunderstorms struck on Tuesday night.
Several people had a lucky escape yesterday when a main road in the village of Drumahoe, outside Derry, collapsed.
A number of people were walking on the road when a section crumbled into flood water.
In Burnfoot around 20 families in the Pairc an Grianan and Lios na Greine estates have been left homeless after their properties were severely damaged by flood water and raw sewage.
Firefighters saved 93 people, with the coast guard helping dozens of others.
Coast guard staff were also involved in the rescues of more than 30 people.
Earlier: Details of humanitarian funding for flood victims in Donegal will be announced today.
Dozens of people spent the night in emergency accommodation after unprecedented rainfall on Tuesday washed away roads, collapsed bridges and destroyed homes.
At least 250 houses have been affected.
Speaking from Donegal, Minister with responsibility for the area Kevin 'Boxer' Moran, said that he is committed to humanitarian relief for residents.
"Everything's gone, ruined, lost. I'm really gutted and very emotional" - Donegal residents react to devastating floods across northwest pic.twitter.com/wRVh0j7ju0— RTÉ News (@rtenews) August 23, 2017
"You can imagine the scale of this damage that we now have to import culverts and look around the country [to see] what's available to us," he said.
"We need to put in the right measures for the long-term gain, particularly for the people of Donegal that have suffered this.
"As regards humanitarian aid for the people, that's something that has to happen."