Built from limestone rubble with dressed quoins, this tower house rises from a distinctive base-batter and survives remarkably intact up to battlement level, though its southwest corner has collapsed above the first floor. The tower measures just over seven metres east to west and six metres north to south, with walls over a metre and a half thick; typical dimensions for a modest Irish tower house. From its elevated position, the castle overlooks undulating terrain that drops away to the east and south, where the remains of its protective bawn wall can still be traced. A ringfort and holy well lie about 100 metres to the north, whilst Knockkelly tower house sits visible on a ridge roughly a kilometre to the southeast, creating a network of medieval and early modern sites across this part of Tipperary.

The castle’s interior reveals the practical defensive architecture common to Irish tower houses. Entry was through a doorway in the east wall, strategically placed beneath a murder hole that allowed defenders to attack intruders from above. The ground floor chamber received light from two windows and contained various domestic features including a wall cupboard, a round-arched recess, and a fireplace that was likely added when a two-storey house was built against the castle’s eastern side in the 17th century. A narrow mural staircase tucked into the south wall provided access to the upper floors, with the first floor featuring a well-preserved barrel vault ceiling. Defensive corbels that once supported angle machicolations survive at the southeast and northwest corners, whilst slop-stones projecting from the south wall at first and second floor levels speak to the everyday life within these walls.

Historical records provide tantalising glimpses of the castle’s past inhabitants. The Civil Survey of 1654-6 described it as ‘a little old Castle with walls of a stone house’, noting that in 1640 it belonged to one Thomas Vynn of Fethard, identified as an ‘Irish Papist’. This designation places the castle within the complex religious and political landscape of 17th-century Ireland, when Catholic landowners faced increasing pressure during the Confederate and Cromwellian wars. The Ordnance Survey Letters mention a chimney that once stood at parapet level on the east wall, now lost to time. Today, while the second floor remains inaccessible and the southwest corner shows significant collapse that has exposed the medieval garderobe chute, Slainstown Castle continues to stand as a testament to the minor gentry who built these modest but formidable homes across the Irish countryside.