The structure, measuring roughly 10.7 metres by 8 metres, survives to two storeys in height, though only its northwest and southwest walls remain fully upstanding. Built from rough limestone rubble without any visible dressed stone, the castle’s walls are about 1.2 metres thick; substantial enough to support the stone vaulted ceiling that once covered the ground floor, traces of which can still be seen where it springs from the inner northwest wall.

The southwest wall, standing about 5 metres high, tells a story of adaptation and decay through its various openings. At ground level, a gap near the western corner likely marks where a window was broken out, whilst directly above at first floor level another possible window opening appears to have met the same fate. Curiously, a blocked window opening is visible in the western corner of the first floor, suggesting modifications to the castle’s defensive or domestic arrangements over time. Much of the castle’s fallen masonry lies within its walls and forms a grass covered mound to the east, whilst an intriguing L-shaped earthen bank extends some 38 metres north from the castle’s western angle before turning east. This earthwork might represent the foundations of a demolished hall that once adjoined the castle’s north face, or perhaps the remnants of a bawn wall that enclosed the tower house.

Historical records paint a picture of a castle already in decline by the mid-17th century. The Down Survey map of 1654-7 shows Heathstown Castle standing beside the medieval road marked as the ‘High way from Dublin to Athlone’, when the lands belonged to the Earl of Clanricard. By 1654, the survey’s terrier rather dismissively noted that the ‘Castle of Heathstowne’ was among ten castles that ‘were very small ones formerly & now quite Demolished’, suggesting that even three and a half centuries ago, this fortified residence had already fallen from whatever modest prominence it once held.