The original six-inch OS map marks the spot plainly enough, labelling it 'Cave' in the north-east quadrant of the rath, a circular earthwork enclosure of the kind built across Ireland during the early medieval period, typically as a defended farmstead. Later editions show nothing at all. The place did not disappear, of course. It simply stopped being noticed.

What lies beneath the Rockfield rath is a souterrain, an underground stone-built passage and chamber of the type commonly associated with Irish raths, most likely used for storage or as a refuge. When the site was inspected in 2021, the entrance was found not in the north-east position marked on the old map but in a copse of hazel and hawthorn in the south-east quadrant of the ringfort. A low, lintel-roofed creep, just 0.9 metres high and 0.7 metres wide, runs for 3.2 metres downward into the main chamber. That chamber is considerably more spacious: 4.8 metres long, 2.7 metres wide, and 2 metres high, built entirely in drystone and roofed with corbelling, a technique in which courses of stone are laid so that each projects slightly inward over the one below, eventually meeting at the top without mortar or keystone. The floor is scattered with loose rubble. At the north-north-east end of the chamber, a narrow recess is set into the wall at roughly knee height, and immediately beside it a small lintelled opening leads into a further passage extending to the north-north-east, though that passage has partly collapsed and cannot be fully explored.