Frightened residents had to flee their homes after a landslide sent thousands of tonnes of sodden peat cascading down a hillside.
The flood of earth travelled for several kilometres near Drumkeerin village, Co Leitrim, burying farmland in layers metres deep in places.
Several roads were also blocked, woodland was engulfed and local rivers and streams have been deluged.
Farmers gathered yesterday to try to assess the scale of the destruction and locate missing livestock.
Councillor Pádraig Fallon, who joined them, said they were all shocked by what they saw.
"It's impossible to describe it unless you were here. We were trying to find where it started but it's further than I can see. We'll need a drone or aircraft to see the full extent of it."
The land started moving on Sunday evening after days of heavy rain followed months of unusually dry weather.
"People said it was like they could hear very loud continuous thunder, but with thunder you'd look up. This was coming at them at ground level and it was very frightening," Mr Fallon said.
"There are a few houses cut off but luckily there's no one in them. The occupants left on Sunday. You couldn't stay there in the dark not knowing how close the slide was going to get or if it was going to come down on top of you."
A landslide caused extensive damage on the opposite side of the valley in 2008 but that incident was attributed to the construction of a wind farm.
No works were being carried out at the location of Sunday's slide which locals said was largely left undisturbed apart from grazing livestock.
Around a dozen farmers are affected and the Irish Farmers Association said it would ask the Department of Agriculture to declare force majeure so that farmers would not have their land declared unproductive and ineligible for farm payments.
Inland Fisheries Ireland said extensive damage was caused. "It may take some time before it is safe to carry out proper assessments," it said.
Leitrim County Council engineering staff have been on site since Sunday. "The council continues to assess the situation and monitor the direct impacts," it said.
Mr Fallon said there were fears of further slides as more rain was forecast.