The roofless remains of a medieval church dedicated to St Patrick, after whom the islet is named. The ruins stand just to the east of a round tower at the highest point of St Patrick's Isle. It may date to the 10th to 12th centuries AD. 


It is thought that the church was also a cathedral and that it was contemporary with similar Irish cathedrals of 10th century date. The oldest parts of the building are contemporary with the adjacent Round Tower, but there has been much reconstruction.  After severe damage, which is perhaps to be connected with the fortification of the island by King Magnus in 1098, the cathedral was rebuilt in herringbone masonry and extended westwards. The east end is of a later date than the rest and has been altered several times. The east two-light, arched window was made into a doorway in the 18th century but was restored in 1929. The west gable and belfry were blown down in the 19th century.


The building overall measures 19 metres in length by 7.5 metres wide, with walls averaging 1 metre in thickness. The remaining walls are largely up to their original heights, with the east gable surviving almost to the peak, and containing a large window, within which no tracery survives.


The masonry shows a complex construction history which is not well understood. It is built mostly of red sandstone, but includes some medium sized slates. At the base it is of medium rubble, quite well coursed. This is succeeded in north and south walls by sandstone courses laid in 'herring bone' fashion. Above this again are more horizontal sandstone courses. The present northern doorway is a later insertion, the older doorway, now blocked, being visible in the middle of the west end.