The 3rd edition Ordnance Survey 6-inch map reveals an oval enclosure that dominates the small island, though the site remains relatively inaccessible to visitors. This stone fortification likely shares characteristics with other island cashels found throughout Donegal, such as the example at site 819, suggesting it was once part of a broader network of defensive structures that dotted the county’s coastal and island landscapes.

Cashels, or stone-built ringforts, served as fortified homesteads during the early medieval period in Ireland, typically housing extended families and their livestock. The choice of an island location for this particular cashel would have provided natural defences, with the surrounding waters of Ballyness Bay acting as an additional barrier against potential raiders. The oval shape of the enclosure, as recorded in historical surveys, follows the typical pattern of these structures, which were built to maximise the use of available land whilst maintaining defensive capabilities.

The site was documented as part of the Archaeological Survey of County Donegal, a comprehensive catalogue compiled in 1983 that traces human activity in the region from the Mesolithic period through to the 17th century. While the cashel on Boyle’s Island awaits further archaeological investigation, its presence adds another piece to the complex puzzle of early medieval settlement patterns along Donegal’s rugged Atlantic coastline, where communities balanced the need for security with access to both marine and agricultural resources.