One of the more fragmentary of these is known simply as Structure 5, a wall so reduced by time and later building activity that it cannot even suggest what kind of building it once belonged to. Only a short stretch of its foundation course survives in the north-western sector, running roughly three metres in length and surviving to a height of less than half a metre. What remains is enough to show that it was built in a mixed technique, with a double-faced central section, meaning two parallel rows of stone forming a proper wall, flanked at each end by single upright slabs called orthostats. Beyond that, the archaeology goes quiet.

The structure was identified and recorded during research excavations in the western half of the inner enclosure, work that has slowly unpicked the layered building history buried beneath Dún Aonghasa. Structure 5 sits in an awkward middle position in that sequence. Its southern wall cut across and partly destroyed the northern wall of an earlier structure, known as Structure 8b, establishing that it came after that building. At the same time, it was already in existence before a later phase of work reinforced or altered the wall of the inner enclosure itself. A further structure, Structure 2, was built after Structure 5, adding yet another layer to the sequence. No occupation surface or floor deposit could be confidently linked to Structure 5, so it remains impossible to say with any certainty what the building was used for or precisely when it stood. The findings are published in Claire Cotter's 2012 study of the site.