Known historically as Castrum Olethan or Caisleán O Liatháin, meaning ‘the castle of Uí Liathán’, this stronghold was built by the Barry family in the 13th century when Castlelyons became one of their principal manors. A devastating fire in 1771 reduced what was then a large square building with a central courtyard to ruins, and subsequent quarrying has left the remains in two distinct sections separated by about 20 metres of abandoned quarry workings.

The southern section preserves the medieval heart of the complex, including a rectangular structure at its southwest corner measuring approximately 5.4 metres north to south and 5.15 metres east to west. This chamber features a plank-centred vault dating to the 13th century, though its southern half has collapsed and the interior is now filled with rubble. The walls, where visible, stand about 4 metres high with thicknesses of 1.35 metres, typical of medieval defensive architecture. Moving eastward, two bays of a south-facing wall survive to two storeys, showing evidence of multiple building phases; the lower courses curve, indicating an earlier structure beneath, whilst the openings were later enlarged for sash windows during the 17th or 18th century renovations.

The architectural details reveal the castle’s evolution through time. A wicker-centred vault with a north-south axis, clearly inserted into an existing chamber, demonstrates 15th or 16th century modifications, whilst a fireplace with a quarried flue in the western wall speaks to later domestic improvements. The northern section, about 20 metres away across the old quarry, contains remains of a 17th century wing, showing how the castle complex expanded and adapted over its long history before the 1771 fire brought its active life to an end.