A Bronze Age fort high over the sea cliff on Slea Head near Ventry on the Dingle peninsula has suffered further deterioration in recent days, it has been confirmed.
The OPW said the fort at Dún Beag, a national monument believed to be more than 2,500 years old, remains closed to the public.
A large portion of the prehistoric promontory fort was said to have fallen into the sea due to recent storm damage.
At least 600ft over the sea, the fort and the cliff area is popular with visitors. However, the headland area, like much of the Kerry coastline, is soft and vulnerable to erosion.
At least 10m of the dry stone structure has fallen in to the sea. The entrance passage to the Bronze Age fort and a drystone souterrain have also collapsed.
The OPW is warning people not to enter the site as it is extremely dangerous and unstable.
“The OPW cannot emphasise enough the absolute dangerous nature of Dún Beag Fort at this time and asks that all visitors, for their own safety, refrain from visiting the site until further notice,” a spokesperson said.