The castle was likely built by the O’Dowlings, one of the seven powerful septs of Laois, before the Crown granted it to John Dunkirley in 1550. By 1576, the property had passed to Robert Hartpole of Carlow, who received an extensive grant that included not just Clonreher (then called Clonrere or Merickeston) but numerous other castles and lands throughout the lordship of Slemarge. Hartpole’s grant came with considerable responsibilities; he was required to maintain six English horsemen and pay an annual rent of £26 1s. 2d. to the Crown, holding the lands by the service of a fourth part of a knight’s fee.

The tower house itself measures 11.75 metres north to south and 9.45 metres east to west, with projecting towers at the northeast and southwest corners that rise an additional storey above the main structure. Built without dressed stonework or the typical defensive base-batter found in many Irish tower houses, the castle’s defensive features are nonetheless impressive. A barrel vault with a loft covers the ground floor, whilst a mural passage runs through the eastern wall at loft level. Access to the upper floors, now inaccessible, was via a spiral staircase tucked into the southeast angle. The southwest tower contains particularly interesting architectural details, including a barrel-vaulted roof constructed using wicker-centring, a medieval building technique, and a garderobe chamber at first-floor level.

Most of the castle’s windows are simple defensive slits, though a two-light pointed arch window in the eastern wall hints at more refined living quarters within. Some modern rectangular windows have been added over the centuries, but the original doorway’s location remains a mystery, lost to time and structural changes. The castle’s relatively austere construction, lacking the ornamental stonework found in grander Irish castles, speaks to its primarily defensive purpose during a turbulent period when the Hartpoles and other English settlers were establishing their presence in what had been firmly Irish territory for centuries.