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Dún Aonghasa is a prehistoric hill fort (Iron age / Bronze age) on the Aran Islands. It is located on Inishmór, at the edge of a 100 metre high cliff.
The fort consists of three walls enclosing an area of 6 hectares.
Outside the third ring of walls lies a defensive system of stone slabs, known as a cheval de frise, planted in an upright position in the ground and still largely well-preserved.
It is a popular tourist attraction and an important archaeological site. It is a National Monument of Ireland .
Wikipedia: Link

A National Monument in the Republic of Ireland is a structure or site, the preservation of which has been deemed to be of national importance and therefore worthy of state protection. If the land adjoining the monument is essential to protect it, this land may also be protected.
A list of the monuments can be found at Link