Local tradition holds that St. Ciaran of Saiger established a nunnery here in the fifth century, though by 1627 the site had been transformed into something quite different. Nicholas Herbert built a fortified house and bawn on what may have been the original religious grounds, creating a defensive structure that reflected the turbulent times of 17th-century Ireland.

What remains today are the weathered footings of an impressive defensive complex. The main enclosure forms a square roughly 52 metres on each side, with traces of circular turrets at the northwest and southeast corners. The northwest turret still stands about three metres high, whilst fragments of the southeast turret connect to a southern wall that features gun loops; clear evidence of the site’s military purpose. Within this courtyard, the foundations of a substantial building measuring 12 by 33 metres run along an east-west axis, likely the main residence of the fortified house.

When antiquarian Thomas Cooke visited in 1875, he could still make out a large square gatehouse that opened onto the courtyard, complete with small round flanking towers bristling with gun loops. By then the site was already known as Killyon Manor, recorded as being occupied by someone named Herbert (or Harbart) in 1634. The Ordnance Survey later marked it as ‘Killyon Abbey’ on their six-inch maps, preserving the memory of its earlier religious use. A datestone from the castle has since been relocated to the parish hall in nearby Rath village, whilst the site itself was entered into the Register of Historic Monuments in December 1988.