The monument consists of a roofless medieval chapel, associated with, and sited on a knoll 150m WSW of, Dunstaffnage Castle, together with a burial aisle added to its E end in 1740 for the Campbells of Dunstaffnage.
The chapel dates to the second quarter of the 13th century, and measures 22.1m E-W by 8.1m N-S across walls 0.9m thick; the burial aisle measures a further 5.4m E-W. There was no structural division between nave and chancel and the chapel was wooden-roofed. The walls are of well-coursed rubble, with pink and buff sandstone dressings. There are remains of 2 doorways in the S wall and one in the N, and remains of 3 symmetrically-disposed windows in each of the N and S walls, one lighting the E end of the nave and 2 the chancel. There were originally 2 windows in the E wall. All the openings are elaborately moulded, with dog-tooth ornament. The external angles have roll-mouldings. Most of the chapel is preserved to near wallhead height, but there are gaps in the N and E walls.
The chapel was built by the owners of the castle, the MacDougalls, but ownership eventually passed from their heirs to the Campbell Earls of Argyll in 1469-70, whereafter hereditary wardenship was awarded to their kinsmen, the Campbells of Dunstaffnage. During the post-medieval period, the interior was used for burials; the only grave slabs now remaining are those in the 1740 burial aisle.
The area to be scheduled comprises the chapel and burial aisle (but excluding the ground within the walls of the burial aisle) and an area extending 10m from their walls in each direction, in which evidence may be present for burials and other activity associated with the use and construction of the chapel, as marked in red on the accompanying map.