This three-storey limestone fortress, built from roughly coursed blocks, boulders and greywacke, originally featured projecting towers at its north-northeast and south-southwest corners, though centuries of modifications have considerably altered its original form. The castle's unusual alignment, not quite following a true north-south axis, adds to its distinctive character amongst the tower houses of County Louth.

The castle's interior reveals a complex history of alterations and additions. Entry is gained through a doorway on the northwest side that leads first into a small bawn area rather than directly into the castle proper; evidence of an earlier, abandoned doorway can still be seen nearby in the north wall. Inside, the ground floor features a barrel vault occupying the western portion, whilst the upper floors showcase increasingly refined architectural elements, including finely worked granite windows with ogee heads, garderobes with corbelled roofs tucked into the southeast corners, and a large fireplace in the northeast angle. The second floor once supported wooden flooring on corbels, and above it all sits a magnificent corbelled stone roof, an unusual solid construction that eliminates the need for a traditional wall-walk.

Archaeological investigations in 2003 uncovered intriguing evidence of the site's longer history, revealing a 21-metre stretch of wall and the remains of a square tower built into the fill of a substantial 13th-century defensive ditch. This discovery suggests either an association with the current tower house or the tantalising possibility of an earlier castle on the site. Now protected as a National Monument under state guardianship, Termonfeckin Castle represents not just a single building phase but rather centuries of adaptation, with its repositioned doorways, added buttresses, incorporated chimney stacks, and eastern annexes all telling the story of a living fortress that evolved with the needs of its inhabitants through the medieval period and beyond.