The Grianan of Aileach ( GREE-nən əv AL-yə(kh); Irish: Grianán Ailigh [ˌɟɾʲiənˠaːnˠ ˈalʲiː]), sometimes anglicised as Greenan Ely or Greenan Fort, is a hillfort atop the 244 metres (801 ft) high Greenan Mountain at Inishowen in County Donegal, Ireland. 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 (15 ft) thick and 5 metres (16 ft) 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.

By the 12th century, the Kingdom of Ailech had become embattled and lost a fair amount of territory to the invading Normans. According to Irish literature, the ringfort was mostly destroyed by Muirchertach Ua Briain, King of Munster, in 1101.

According to Tony Nugent, the Grianan was also used as a Mass rock during the anti-Catholic religious persecution that began under Henry VIII and ended only with Catholic Emancipation in 1829.

Substantial restoration work was carried out in 1870. Today, the site is protected as a national monument and is a tourist attraction.