The structure measures approximately 10 metres from northeast to southwest and 11.8 metres from northwest to southeast, with walls an impressive 2.35 metres thick. Built from roughly coursed limestone, the castle's construction shows particular attention to detail in its finer elements; punch-dressed limestone blocks with carefully finished margins form the quoins, whilst similar craftsmanship is evident in the window and doorway surrounds.

The interior layout reveals the defensive and practical considerations typical of Irish tower houses from this period. A stone groin vault spans the second floor, providing both structural support and fire protection for the upper levels, whilst the remaining floors would have been constructed from timber. The castle's builders incorporated several functional features into the thick walls themselves, including garderobes tucked into the northeast corner, a mural passage running through the southwest wall, and separate mural staircases in both the southwest and southeast walls, allowing for movement between floors without sacrificing valuable internal space.

Tinnakill Castle has an intriguing connection to one of Ireland's more mysterious medieval carvings. A sheela-na-gig, one of those enigmatic female exhibitionist figures found on churches and castles throughout Ireland, is said to have originated from this site. These stone carvings, which date from the medieval period, continue to puzzle historians and archaeologists regarding their exact purpose and meaning. The castle was documented in the Journal of the Kilkenny Archaeological Society in both 1904 and 1920, cementing its place in the recorded heritage of County Laois.