This ancient stronghold, known in Irish as Dun Masc after Masg, son of the fourth son of the King of Leinster, served as the seat of power for the O’More family, Kings of Leix. The site’s strategic importance meant it changed hands numerous times throughout its tumultuous history; from Diarmait MacMurchada gifting it to Strongbow as part of his daughter Aoife’s dowry in 1170, to its eventual destruction by Cromwellian forces in 1650. Between these dates, it passed through the hands of Roger de Mortimer, was granted to the Earl of Thomond in 1609, and witnessed dramatic sieges during the Confederate Wars when it was captured by Sir Charles Coote in 1641 and recaptured by Eoin Rua O’Neill in 1646.

Archaeological investigations have revealed that the castle developed over several distinct phases, with the rectangular keep dating from around 1220, possibly even as early as 1180. The fortification expanded outward from the summit in stages; the inner ward with its defensive walls and mural towers came first, followed by the middle ward with its twin bastioned gatehouse, and finally the outer ward and its own gatehouse, both constructed during the thirteenth century. The entire complex stretched across the rock outcrop, with the external triangular bailey measuring roughly 80 by 60 metres internally. Access to the outer ward was originally via drawbridge across a deep rock cut ditch, later replaced by a stone causeway that visitors can still see today.

Recent excavations by the Office of Public Works have uncovered fascinating details about the castle’s construction and modification over time. Work around the barbican gatehouse revealed evidence of a drawbridge pit dating to the mid to late thirteenth century, whilst investigations at the inner gatehouse discovered an earlier rectangular stone gateway that predated the main fortifications. This earlier structure, measuring 10 by 8 metres, was eventually incorporated into the expanding castle complex and repurposed for various uses including, at one point, a smithy. The keep underwent substantial remodelling around 1600, making it the only part of the castle significantly altered in the seventeenth century. After its dismantling in 1650, Sir John Parnell attempted to bring new life to the ruins in 1795 by constructing banqueting halls and other buildings, though these too have long since fallen into decay.