The site began as a 12th-century earthwork castle, likely a motte topped with timber defences, before evolving into a substantial masonry fortress by the 13th century. The Barony of Carbury was initially granted to Meiler FitzHenry but later came into the possession of William Marshal, with the stone castle first appearing in records by 1234. The de Bermingham family, who erected the castle reputedly on the site of ‘Ua Ciardha’s fort’, became notorious for their brutal politics; in 1305, Peter de Bermingham orchestrated the massacre of approximately 30 leading O’Connors of Offaly and their followers within these walls, sparking widespread warfare between the Anglo-Normans and the Gaelic Irish across Leinster and Desmond.

The castle’s architecture reveals its complex evolution through centuries of conflict and reconstruction. The original Anglo-Norman masonry structure survives as a sub-rectangular building with walls nearly two metres thick, standing three storeys high and originally accessed through an imposing doorway approached by a stone-revetted ramp. In the late 13th century, three parallel barrel vaults were inserted into the structure, though they weren’t bonded to the original walls. Later extensions to the north and east transformed the medieval fortress into a late 16th or 17th-century fortified house, complete with Jacobean-style chimneys and mullioned windows. The steep-sided earthwork motte, measuring about 26 metres across and rising five metres high, may have been repurposed as a bawn or bailey for the stone castle, with remnants of angle towers and retaining walls still visible at its base.

The castle’s turbulent history continued through the centuries, with Lord Furnival rebuilding it in 1447, only to see it demolished in 1475 and again plundered and burnt in 1556. In 1562, ownership passed to the Colley family, ancestors of the Duke of Wellington, who maintained the property until it was finally abandoned in 1774. Today, the ruins stand prominently on Carbury Hill at 470 feet above sea level, overlooking bogland to the north and east, with three ancient barrows visible on the higher ground to the south. A church and graveyard approximately 80 metres downslope may have medieval origins, adding another layer to this historically rich site. The vertical quarry face along the western and northern edges of the motte suggests that stone for the castle’s construction was sourced directly from the hill itself, a practical solution that simultaneously strengthened the site’s defensive position.