Sliabh Liag in Co Donegal has been closed to the public by gardaí who are investigating an incident.
A garda operation has been under way all day at the cliffs in the south of the county.
The Irish Coast Guard has confirmed its Rescue 118 helicopter was assisting gardaí in what it said was an ongoing missing person's search.
Gardaí say their investigation is ongoing and the scene will be preserved overnight.
They say the cliffs will continue to be inaccessible to visitors again tomorrow.
