Grianán of Aileach also known as the Greenan Fort is located at the summit of Greenan Mountain in Inishowen, Co. Donegal, Ireland. Looking out over Lough Swilly and Lough Foyle the fort provides breathtaking panoramic views of counties Donegal, Derry, Tyrone and Antrim.
In Irish folklore, the original ringfort is said to have been built around the grave of Aedh, son of the Dagda, a god and the celebrated king of the Tuatha Dé Danann. The hillfort is one of only 5 Irish sites marked on Ptolemy of Alexandria’s 2nd century map of the world. The existing fort was probably built in the early Christian era. It became the royal seat for the northern Ui Néill dynasty from 789 to about 1050. It was mostly destroyed by Murtagh O’Brien, king of Munster in 1101, in reprisal for the Ui Neill’s destruction of his royal seat at Kincora. The king is said to have ordered each of his soldiers to take away a stone from the fort as they departed.
George Petrie who first surveyed the Grianán of Aileach in the 1830’s, described the ruins as a circular wall enclosing an area of 23.6 metres (77ft) in diameter. Standing 1.8 metres (5ft 11in) high, the wall varied in breadth from 4.6m (15ft) to 3.5m (11ft). One entranceway led through the wall to the interior where flights of steps on either side accessed terracing. Fallen stones had covered any other existing stairs. Petrie believed that the wall was originally between two and four times higher with three or four terraces ascending to the top of the wall. He also recorded the remains of a small oblong church measuring 5 metres (16ft) by 4.3 metres (14ft) within the fort.
Outside the fort he noted an ancient road between two ledges of natural rock which led to the entrance. Petrie’s plan of the site shows a line of stones leading up to the entrance. These are now gone. The cashel is surrounded by three concentric ramparts dating from the late Bronze Age or Iron Age, enclosing an area of about 5 acres. Between the two outer banks on the south side of the hill, is the formerly covered spring well which is dedicated to St. Patrick. Petrie describes a prehistoric burial mound or tumulus between the second and third wall, as being a small mound surrounded by a circle of ten stones. These stones were laid horizontally and converged towards the centre. Possibly dating from the Neolithic period (about 3000 BC), it was subsequently destroyed and its position is marked by a heap of broken stones.
Grianán of Aileach was reconstructed by Dr Walter Bernard of Derry from 1874 to 1878. It was found that parts of the original drystone masonry had been preserved under collapsed stone. During the restoration the workers marked the undisturbed portions and used the collapsed stone to build on this foundation. They supplemented these with other stones from the area to replace those ‘removed by King Murtagh O’Brien in 1101’. The lintel over the passageway was replaced. The repaired wall now rises in three tiers in the interior to a height of about 5m.