The name Donegal itself carries layers of meaning -- it translates as "Fort of the Foreigner," possibly referencing a Viking fortress in the area that was destroyed in 1159. No archaeological trace of that Norse fort survives beneath the centuries of rebuilding. The castle as it stands was built by Hugh Roe O'Donnell I in the 1460s or early 1470s. Hugh Roe was the wealthy chief of the O'Donnell clan, Lords of Tir Conaill, one of the most powerful Gaelic families in Ireland for over a millennium. In 1474, he and his wife Nuala founded a Franciscan friary further down the river. Local legend speaks of a tunnel connecting castle and friary, though no evidence has been found. The castle sits on a bend in the River Eske near the mouth of Donegal Bay, its walls built from locally quarried limestone with some sandstone.

The O'Donnells fought the Nine Years' War against English rule from 1594 to 1603, and lost. In 1607, the leaders of the clan joined the Flight of the Earls, abandoning Ireland for the Catholic courts of continental Europe. They never returned. Before they left, the O'Donnells severely damaged the castle's tower house -- a deliberate act of sabotage to prevent the English from using it against the remaining Gaelic clans. In 1611, during the Plantation of Ulster, the castle and its lands were granted to an English captain named Basil Brooke. He quickly repaired the tower and then transformed it, adding windows, a gable, and a large manor-house wing in the Jacobean style. The result is the architectural palimpsest visible today: a 15th-century Gaelic keep married to a 17th-century English manor, one family's fortress made over into another family's country house.

The Brooke family held the castle until the 1670s, when they moved to Lisnaskea in County Fermanagh and sold the property to the Gore dynasty, who later became Earls of Arran. Under the Gores, the castle fell into ruin during the early 18th century. Two hundred years of neglect followed. By the time the 5th Earl of Arran vested the property in the Office of Public Works in 1898, the buildings were skeletal. The castle had gone from being the greatest fortress in all Ireland to a roofless shell in the center of a small market town -- a trajectory that mirrors the broader story of Gaelic Ireland's great houses.

In the early 1990s, the Office of Public Works undertook a careful restoration. New roofing and flooring were added to the tower house using techniques consistent with the 15th- and 17th-century originals. Some of the oak timbers came from the Colebrooke Estate near Brookeborough in County Fermanagh -- a distant echo of the Brooke family who had first rebuilt the castle after the O'Donnells destroyed it. The manor wing was partially reroofed, the stonework repaired, and the exterior of the tower house harled in the traditional manner. Today the castle hosts Gaelic cultural evenings and Ulster Scots events, the descendants of both traditions gathering under a roof that neither the O'Donnells nor the Brookes could have imagined sharing.