Built around the mid-fifteenth century, this oblong tower house measures 14 metres by 9.1 metres and originally comprised five storeys, with the main wing featuring four wooden floors beneath a stone vault. The defensive architecture includes strategically placed skew-loops on the second and third floors, ogee-headed window slits, and a spiral staircase providing access to each level. Though the southern face and much of the side walls have collapsed over the centuries, the remaining structure still offers insight into the defensive priorities of its builders, with numerous ambries and vaulted chambers throughout.

The castle’s turbulent history reflects the broader conflicts of early modern Ireland. In 1583, Hugh Wall of Dunmoylin owned the property before joining a rebellion, after which ownership passed to Oliver Stephenson of Dunmoylin in 1615, with confirmation to his son Richard. The Stephenson family, identified as Irish Papists in the Civil Survey of 1654-56, held the castle until its confiscation in 1655 during the Cromwellian period. By the time of the survey, both castle and bawn were already described as ruined, suggesting the fortification had fallen into disrepair even before its final confiscation.

Perhaps most intriguing is the castle’s defensive bawn, a rectangular enclosure measuring 19.2 metres east to west and 25 metres north to south, with walls that originally stood 0.9 metres thick at the top, broadening to 1.8 metres at the base through battering. Three of the original four corner turrets survive; these D-shaped flankers, between 4 and 5 metres in external diameter, featured vaulted roofs and measured 3.65 metres internally. One turret still retains its vault and three gunloops, testament to the evolution of castle defences in response to gunpowder warfare. Archaeological classification places Garraunboy as a Type 1A tower house, characterised by its entrance lobby system that provided separate access to the spiral staircase, a subsidiary chamber, and the main chamber, a layout that balanced defensive needs with domestic functionality.