Yet that surviving scrap encodes a more complicated building than the ruin suggests. The original church measured roughly 26.6 metres east to west and 4.9 metres north to south, built from randomly coursed limestone and sandstone blocks, but the western end was substantially widened to 9.2 metres to accommodate a priest's residence, a chamber with walls 1.5 metres thick to bear the weight of an east-west barrel vault overhead. According to O'Flanagan, writing in 1933, this western addition rose to two storeys, with a staircase tucked into the north-west corner and the main entrance set in the west gable. The site sits on a slight rise in upland ground, with a holy well to the north-west and the earthen bank of a possible graveyard enclosing the southern sector where the ruins stand.

The most telling detail about Letter is not what has survived on site but what was carried away. When Cadamstown church was being built in the nineteenth century, two carved stones from Letter were reused in its fabric. Both are Romanesque in date, a style of rounded-arch ornamental carving common in Irish ecclesiastical architecture from roughly the twelfth century onward. One is a corbel, a projecting stone bracket, carved with a human head; the other is a hood moulding terminal of the Hiberno-Romanesque type, the decorative end-piece of the arched frame above a doorway or window, comparable in style to surviving examples at Clonmacnoise a short distance to the north. During more recent restoration work at Cadamstown, an Early Christian cross-slab from Letter was also found built into the west gable. A further Romanesque fragment, possibly from the same church, turned up on top of a gate pillar at Cadamstown House. The cumulative picture is of a site that was once architecturally ambitious, its carved stonework dispersed across the local landscape over the course of two centuries.