Whether they knew it or not, they were following the line of a structure already more than a thousand years old. That coincidence, or perhaps a deliberate choice to use surviving stonework, gives this site an odd layered quality: early medieval construction running beneath later funerary architecture, both now partially obscured by collapsed earth and overgrown ground.

The structure at Lydacan is a souterrain, an underground passage or chamber built from drystone walling and roofed with large lintels, typically associated with early medieval ringforts and used for storage or refuge. This one sits within the south-western quadrant of a rath, the Irish term for a roughly circular earthen enclosure that served as a farmstead during the early medieval period. The souterrain is L-shaped in plan and extends to more than 16.8 metres in total length. Its long axis runs north-north-east to south-south-west for 7.8 metres before turning sharply to the south-west. Only the north-north-eastern end of the chamber survives intact, measuring 4.3 metres long, 1.35 metres wide, and 1.35 metres high. A partly blocked recess or creep, the small low openings sometimes built into souterrain walls to allow movement between sections or provide concealment, is still visible in the eastern side-wall. The rest of the passage has largely collapsed and is now readable only as a linear depression in the ground. At the western end of that depression, the 19th-century burial vault, measuring 3 metres in length, appears to have been built directly on the souterrain's existing line.

The accessible section is entered roughly midway along the structure, where a roof lintel has fallen. The surviving chamber is low and narrow enough that movement inside is awkward, and much of the rest of the underground course is traceable only by the shallow surface depression it leaves. The burial vault at the far end adds an additional complication to reading the site; the two phases of construction have become entangled in a way that makes it genuinely difficult to tell where early medieval drystone work ends and later masonry begins.