Local tradition, recorded by historian Hugh Allingham in 1879, suggests that the fort was built in 1798 on the site of an ancient earthen mound, though this dating has proven contentious. The antiquarian John O’Donovan claimed that both an ancient fort and the mound of Aedh Ruadh, or Red Hugh, were destroyed during the fort’s construction, leading to the area becoming known as Fort Hill.

Historical evidence points to a much earlier origin for the fortification, possibly dating to 1597 during Sir Conyers Clifford’s military campaign against the O’Donnells. As de facto Crown governor of Connaught, Clifford marched north that July to subdue the powerful Irish clan. After camping at Assaroe, he repositioned his forces to Mullaghnashee, where Tibbot ne Long Burke had arrived by sea from Galway with provisions and two small field pieces. From this elevated position, Clifford’s artillery bombarded Ballyshannon castle for three days, though the assault ultimately failed; shortage of food and gunpowder forced him to abandon the siege and retreat south via Assaroe, suffering heavy casualties along the way.

Today, little remains visible of the fort beyond the eastern and southern earthen banks in the field immediately west of the church, though aerial photography reveals traces of additional defensive features. The site represents a palimpsest of Irish history, where ancient earthworks gave way to early modern military engineering, each layer adding to the complex story of contested territory in County Donegal.