The structure is a souterrain, an underground passage or chamber built without mortar, typically during the early medieval period in Ireland, and often associated with the ringforts, or raths, that dot the Irish countryside. Most souterrains are modest affairs, a single passage, a low chamber. The one at Dromorehill is something more involved.

Situated in the north-west quadrant of its associated rath, the souterrain stretches to more than 17.4 metres in total length and comprises three, possibly four, distinct passages connected by narrow crawl-through openings known as creeps. The first passage runs roughly south-south-east to north-north-west and is now entered through a modern breach in its wall. A creep at the north end of its western side-wall, though now blocked, once led into the second passage, which runs parallel to the first and stands approximately 1.8 metres high at its tallest measured point. A slightly dog-legged creep at the northern end of the second passage then opens into a third, which changes orientation entirely, running east-north-east to west-south-west. The deliberate changes in direction, combined with the tight creeps between sections, are thought to have made such structures easier to defend or conceal. What makes the Dromorehill souterrain particularly interesting is that a large natural boulder has been incorporated directly into the north side-wall of this third passage, worked into the drystone construction rather than removed or avoided. Beyond that boulder, a further possible passage of around three metres continues northward, running beneath the bank of the rath itself, though its full extent remains uncertain.