Grianan of Aileach, National ringfort in County Donegal, Ireland

Grianan of Aileach is a dry-stone ringfort crowning Greenan Mountain, its wall about 4.5 m thick and 5 m high, enclosing a circular interior 23.6 m across. Three terraces inside are reached by steps, and two long passages run within the wall toward the lintel-covered entrance on the east. The present masonry is largely a reconstruction of 1874–1878.

The site was used before the stone fort was built; a tumulus nearby may date to the Neolithic, and a covered well was found beside the cashel in the early 19th century. The Northern Uí Néill are credited with the ringfort in the sixth or seventh century, and it became the seat of the Kingdom of Ailech. Muirchertach Ua Briain, King of Munster, largely destroyed it in 1101. George Petrie surveyed the ruin in the 1830s, and restoration followed in 1874–1878.

The site connects to Irish mythology through stories of the god Dagda commissioning its construction as a burial monument for his son. These mythological associations shape how people understand and experience this place today.

The site can be explored on foot with informational panels explaining the architectural features and surrounding archaeological remains visible in the landscape. Weather protection is important as the location sits exposed on a hilltop where conditions can be quite windy.

George Petrie's 1830s survey found the cashel in ruins, its wall only 1.8 metres high; he estimated it had once stood two to four times taller. During the 1874–1878 restoration, workers marked surviving original drystone masonry with tar and rebuilt on those foundations using fallen stone.