This impressive fortification measures 11.25 metres from northeast to southwest and 9.3 metres from northwest to southeast, with walls an imposing 2.4 metres thick. The structure showcases the defensive architecture typical of medieval Ireland, complete with dressed quoins, a slight base batter, and a bartizan projecting from the eastern angle. Machicolations on the northwest and southwest walls at third floor level allowed defenders to drop stones or boiling liquids on attackers below, whilst the narrow window slits and occasional two-light ogee-headed openings provided both defence and modest illumination.

The main entrance on the southwest wall reveals sophisticated stonework, featuring a two-centred arch constructed from limestone blocks that were hammer-dressed with fine drafting and chamfering. This doorway, equipped with yett holes for securing a protective iron grille, leads into a carefully designed defensive layout. The entrance lobby is covered by a cross loop in the northeast wall, allowing defenders to fire upon unwelcome visitors. Beyond this, a second lobby provides access to both the main ground floor chamber and a spiral stairwell tucked into the southern angle. Historical accounts from 1867 describe an intriguing feature: sundials marked on ten steps of the stairs, which decreased in size from top to bottom, suggesting the tower’s inhabitants had a practical interest in timekeeping.

The interior arrangement reveals the typical domestic needs of a fortified residence. Mural passages run through the northwest wall at first and second floor levels, whilst rectangular chambers sit directly above the entranceway on each floor. Fireplaces warm the second floor from the southeast wall and the third floor from the northwest wall, indicating these were likely the main living quarters. Unlike many tower houses of the period, Ballagh Castle lacks a barrel vault, relying instead on timber floors between levels. Earthworks to the southwest of the tower suggest the possible remains of a bawn, the walled courtyard that would have provided additional protection and space for livestock and storage buildings.