The site offers commanding views to the northwest and southeast, with a stream gully marking the northeastern boundary and a medieval church and graveyard standing about 300 metres to the north. When antiquarian John O'Donovan visited the area in 1838 whilst compiling his Ordnance Survey Letters, locals told him of a castle that once stood in Tulranhan townland, though no trace of it remained even then, and the OS six-inch maps never marked its location.

It wasn't until 1911 that historian H.T. Knox identified what he believed to be the castle's remains; a series of earthworks straddling the townland boundary. Knox documented a mound measuring roughly nine metres square at its base and rising about 2.5 metres high, which he proposed had once supported a stone castle dating to the sixteenth century. He also noted several low, subrectangular earthen platforms surrounding it, which he interpreted as baileys, the defensive courtyards typical of medieval fortifications.

Today, visitors will find no standing walls or obvious castle ruins, only grass-covered earthworks and a roughly square mound incorporated into the townland boundary wall on its southern and western sides. The mound, now only 1.6 metres at its highest point, has a flat top measuring approximately 5.6 by 5 metres; a surprisingly modest footprint for what Knox believed was once a castle. Whether this really was the site of the lost MacCostellos stronghold remains a matter of speculation, but the earthworks continue to mark this boundary between townlands, preserving a tantalising link to Mayo's medieval past.