A large section of Mt Brandon in Co Kerry has slipped into the sea.
The landslide in the uninhabited Sauce Creek — An Sás — between Brandon Head and Brandon Point claimed a section of the mountain about a quarter of a mile long.
The mountain is a favourite with climbers and features a pilgrimage route honouring St Brendan.
Valentia coastguard station said it was conducting an investigation after it received reports in the morning from sea traffic about "rocks falling off Brandon".
In recent weeks, locals who climbed Mt Brandon said they had noticed rock falls and small landslides.
The area is fished by lobster boatmen but is also well known as a landmark for passing sailors between Tralee Bay and Dingle.
Most of Kerry’s coastline is reportedly in a "soft" and potentially dangerous condition and subject to erosion.
Five years ago, a whole kilometre of roadway collapsed into the sea at Slea Head on the peninsula.
There has also been dramatic erosion at Inch and Rossbeigh.
County council officials have warned that of the 684km of Kerry coastline, 378km is soft and at risk of erosion.
Its upland areas are also vulnerable because of their large coverings of peat or bogland.
Landslides and erosion are accelerated during high rainfall such as that experienced this month.
According to council estimates, €30m would be needed to address the coastal erosion problem in Kerry.