Located in County Donegal, this National Monument (number 140) encompasses not just the prominent fort but also the remnants of an ancient road, a holy well, and what’s believed to be a tumulus; all testament to centuries of human activity at this strategic hilltop location.

The cashel itself has undergone significant changes since antiquity. When the antiquarian George Petrie surveyed the site in 1835, he documented not only the circular stone structure but also traces of what appeared to be a rectangular building at its centre, which he interpreted as a chapel from the Penal era of the 18th century. This detail speaks to the site’s continued religious significance even during times of Catholic suppression in Ireland. By the time of Petrie’s visit, however, the ancient fort was already showing its age, and within decades it had deteriorated considerably.

The structure visitors see today owes much to Dr. Walter Bernard of Derry, who undertook an ambitious restoration project between 1874 and 1878. Whilst his work has been both praised for preserving the monument and criticised for potentially obscuring original features, it has ensured that the Grianán of Aileach remains accessible to those seeking to connect with Ireland’s pre-Christian and early Christian past. The level interior of the cashel, where Petrie once traced those mysterious foundations, continues to draw visitors who come to experience this atmospheric site where pagan fortress, Christian sanctuary, and Penal Mass station converged across the centuries.