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The Grianan of Aileach is a hillfort on top of the 244 metres high Greenan Mountain at Inishowen in County Donegal. The main structure is a stone ringfort, thought to have been built by the Northern Uí Néill, in the sixth or seventh century CE; although there is evidence that the site had been in use before the fort was built. It has been identified as the seat of the Kingdom of Ailech and one of the royal sites of Gaelic Ireland. The wall is about 4.5 metres thick and 5 metres high. Inside it has three terraces, which are linked by steps, and two long passages within it. Originally, there would have been buildings inside the ringfort. Just outside it are the remains of a well and a tumulus.
According to Irish literature, the ringfort was mostly destroyed by Muirchertach Ua Briain, King of Munster, in 1101. Substantial restoration work was carried out in 1870. Today, the site is an Irish National Monument.
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