This complicated site has attracted human occupation for over 6,000 years. Most of the standing buildings are less than 1,000 years old, however, and can be broadly divided into military and religious uses.


There are no visible prehistoric remains, but excavations have found Mesolithic and Neolithic artefacts, and Bronze and Iron Age structures.


Large numbers of lintel graves show that the island was used for Christian burial from the early Medieval period onwards. For a brief period during the 10th century pagan Viking settlers were buried amongst other burials without grave goods, implying that in some respects the settlers had integrated with the local population.


The cemetery was associated with a small monastic community which was responsible for the construction of St Patrick's Church, St Patrick's Chapel and the Round Tower. The latter is 15m high and is similar to towers found on monastic sites in Ireland. The raised doorway is consistent with its use as a refuge, though the corbels and battlemented parapet above the windows were added as defences were improved. The masonry of St Patrick's Church tells a complex structural history extending from the 10th to 15th centuries.


With the arrival of the Vikings at the end of the 8th century, St Patrick's Isle gradually changed its primarily ecclesiastical focus to one combining church, domestic and military activities. Little evidence for these early changes survives above ground, having been superseded by later defences. Nevertheless, excavations revealed a stone rampart which defended the islet between about 1000 and 1200. This may have been associated with a sophisticated, rectangular, timber-framed house which was built in the 1100s a little to the north of St German's Cathedral.


The cathedral was begun in the 13th century, probably not long before St Patrick's Isle was granted to the Church. The cathedral was drawn into the fortifications that became necessary to defend the island, so that its construction was not completed until the start of the 15th century and resulted in the raising of the central tower and the placing of battlements on it and the transepts. A stair turret in the angle of the nave and the south transept interrupted the easternmost arch of the arcade, and required a new doorway into the south transept further to the south. The south aisle was rendered redundant by these alterations and so was demolished and the arcade blocked up.


The visible stone defences were begun in the late 1300s and involve several defensible towers, which were later linked by a curtain wall. Additional accommodation was provided north of the cathedral, initially for churchmen but later taken over by the military.


The defences of St Patrick's Isle were heavily remodelled during the period 1530-1650. A circular gun battery was constructed at the northern end of the castle and a half-moon battery was built west of the cathedral. The latter was superseded in the mid 17th century by an earthwork fort in the centre of the castle and loopholed walls for musketeers.


Finally, in the late 18th century, a gun battery was placed behind the curtain wall overlooking the site of the modern lifeboat station. This was replaced in the early years of the 19th century with the two-gun battery or 'barbette' now visible.