Part of Benwiskin mountain in north Sligo has been washed down nearly 100m (328ft) after water trapped in a hollow, swollen by six weeks of heavy rain, burst its banks.
A mudbank up to 15m high has blocked a narrow road about 300m up the mountain in the scenic Gleniff Horseshoe Valley.
The mudslide yesterday was several hundred metres away from the nearest homes.
Irish Cattle and Sheep Farmers Association Sligo branch chairman Gabriel Gilmartin, who lives close to the scene, said: "There has been unrelenting rain here for the past six weeks. Something had to give. It's so lucky there was nobody driving along the road when it happened."
The road is used mainly by a handful of local farmers, RTÉ engineers on their way to the nearby Truskmore transmitter and, in summer, by tourists.
County councillor Patsy Barry, former chairman of Sligo County Council, said the council had agreed to clear the road.
Officials had told the farmers they would have to check the legal position, "but that was only because the council could only clear the road if it had permission to clear the rubble on to the adjoining lands and that has now been agreed".