Round tower, built with evenly coursed, well-finished sandstone masonry with three external string courses and a distinct batter.The tower is 29.2m in height and has a circumference of 15.8 m at the base (Barrow).

The earliest mention of the tower is in 1642 when it was occupied by Irish forces and besieged by the English. Gough (1779) mentions that the interior was white and plastered 'as if newly done' and that three oak beams were still in place under the cap from which the bell was hung. He further describes how the two sill grooves were used for the bell ropes, which were pulled from the ground below. The capstone fell in the 1860s, half of it remains in the cathedral. During internal excavations in 1841 two skeletons were discovered under a flagged and concrete covered floor. Both Gough and Montmorency-Morres (1812) mention that the roof was originally capped with a stone finial 'like a shoe'.

The round tower at Ardmore is unique in Ireland for its internal carved corbels and external string courses, although Lalor (1999) has pointed out a parallel in the use of string courses on the towers at Cormac's chapel, Cashel. The masonry of the tower is of exceptional quality, far exceeding that of the adjacent cathedral. The string courses have a different profile to those in the nearby cathedral, suggesting that the tower predates the cathedral. The current roof and finial cross are modern replacements.