Grianan means a sunny place, or a place with a view. Aileach probably derives from ail, the old Irish word for rock or boulder - so the full name renders roughly as the Stone Palace of the Sunny View. Both halves are accurate. The fort sits 244 metres above sea level on Greenan Mountain at Burt, on the western edge of a small group of hills between the upper reaches of Lough Swilly and Lough Foyle. From the summit you can see across the borders of three modern counties - Donegal, Tyrone, and Londonderry. The hill itself is not particularly tall. What makes the position commanding is its isolation, its panoramic view, and the way it sits at the geographical pivot of a kingdom that controlled trade and political power across northwest Ireland. Inside the cashel - the circular stone wall, about 23 metres across - three terraces ascend, linked by stairs. Two passages run inside the wall, apparently to nowhere, their original purpose still debated.

From the middle of the sixth century until 1101, the Grianan of Aileach served as the capital of the northern Ui Neill. The Tripartite Life of St Patrick records that Patrick blessed the fortress and left a symbolic flagstone, prophesying that many kings and clerics would come from the place. The flagstone is gone, possibly preserved at Belmont House School in Derry as St Columb's Stone, with two feet marks carved on its 2-metre face. The annals report attacks. In 674, Finsnechta Fledach, king of Ireland, destroyed an earlier hillfort on the site - possibly the prehistoric earthworks whose remains still surround the cashel. In 937, Viking raiders demolished it during the reign of Muirchertach mac Neill. In 1006, Brian Boru himself marched through the territory of the Cenel Conail and the Cenel Eogain on a tour of his kingdom. After 1101, the kings of Aileach were kings in name only - the capital was a ruin, the political world had shifted, and the dynasty was negotiating a different Ireland.

By the 1830s, when the Irish antiquarian George Petrie surveyed the site, the cashel had collapsed to a mere ruin standing perhaps 1.8 metres high. Walter Bernard, a Derry doctor, undertook the reconstruction between 1874 and 1878. Bernard faced a problem: he did not have a model. The Grianan was unusual - a multivallate hillfort cashel in a region of Ireland with very few such structures - and Bernard concluded that he would have to look elsewhere for guidance on what the original might have looked like. He chose Staigue Fort in County Kerry, hundreds of miles away in the southwest. From Staigue, Bernard derived the architecture, the proportions, the terraces, much of the visible character of the reconstructed cashel. Whether the actual ancient builders in Donegal had ever built anything like Staigue Fort is impossible to know now. The reconstruction was poorly engineered: within thirty years of state custody, parts of it were crumbling again. What stands today is Bernard's best guess at what an antiquarian thought a Donegal hillfort ought to look like, perched on stones that genuinely are old.

Even after its destruction, the Grianan refused to be merely ruined. During the centuries of anti-Catholic persecution that ran from Henry VIII to Catholic Emancipation in 1829, the Grianan served as a Mass rock - a place where outlawed Catholics could gather to celebrate the liturgy in defiance of the Penal Laws. The penal church Petrie recorded inside the cashel, with walls of mortar barely 62 centimetres high, may date from this era. Outside the walls, on the south side of the hill between two outer banks, sits St Patrick's Well - a covered spring that ties the site to the saint who, according to legend, blessed the fortress. A tumulus, possibly Neolithic, sits between the second and third ramparts: a mound of stones whose meaning was lost long before Petrie's survey. The Grianan now operates as a national monument, a tourist attraction with a small car park and an interpretive sign. Tour groups arrive year-round. The sunset views, when the cloud cooperates, still justify the old name. The Stone Palace of the Sunny View has outlasted its kings.