Local tradition holds that St. Ciaran of Saiger established a nunnery on this spot in the fifth century, though the visible remains today date from much later. What visitors encounter are the grass-covered foundations of what was once an impressive fortified house and bawn, built by Nicholas Herbert in 1627, possibly on the very site where the ancient nunnery once stood.

The surviving ruins paint a picture of a substantial defensive structure from the late medieval period. The footings trace out a large walled enclosure measuring 52 metres square, complete with the fragmentary remains of circular turrets at the northwest and southeast corners. Gun loops piercing the southern wall, still visible today, speak to the turbulent times that necessitated such fortifications. Within this courtyard, the foundations of a long building, 33 metres east to west and 12 metres north to south, mark where the main residence once stood. When antiquarian Cooke visited in 1875, he could still make out a large square gatehouse that opened onto the courtyard, flanked by small round towers bristling with defensive gun loops.

The site’s transformation from religious house to fortified manor reflects the changing fortunes of Irish land ownership during the plantation period. Known as Killyon Manor by the 1630s, when records show it was occupied by someone named Herbert (or Harbart), the building’s military features suggest it was designed as much for defence as for domestic comfort. Though the walls have crumbled and a datestone from the castle now resides in the parish hall at nearby Rath, the ruins continue to intrigue visitors and archaeologists alike; the site was formally entered into the Register of Historic Monuments in December 1988, ensuring its protection for future generations to explore and interpret.