Known locally as An Chloch Ghlas, these lime mortared stone ruins appear on historic Ordnance Survey maps simply as ‘Old castle’, though their true origins remain elusive. When antiquarian John O’Donovan visited in 1835, locals could only speculate that the structure might have been built by the Danes, whilst noting that a fair had been held near the site until around 1755, when it relocated to the growing village of Dungloe proper.

Archaeological surveys conducted in the 1980s and 1990s revealed an irregular, polygonal structure measuring approximately 15 by 16 metres, with walls surviving to just half a metre in height. Rather than a traditional castle with substantial foundations for a roofed building, the remains suggest this was more likely a redoubt or defensive fort. A raised area at the northeastern angle may have served as a gun platform, whilst a ramped entrance can still be traced at the southeastern corner. The 1654 Civil Survey refers to it as ‘ye old castle of Duncloe’, and by 1789 it appeared on maps as a towered building guarding the approaches to Rutland Island.

Some scholars propose the fortification may have been a Mac Sweeney stronghold during the 16th century, though documentary evidence is lacking. Intriguingly, historian Dónall Mac Giolla Easpaig suggests the site may have even older roots; the name Dungloe itself possibly derives from *Dun gCloiche, indicating an earlier cashel or stone fort may have occupied this strategic position before the current structure was built. Whatever its precise history, this atmospheric ruin continues to keep watch over its small bay, its crumbling walls a testament to centuries of coastal defence and local memory.