The circular earthwork here, known as a rath, measures just under 29 metres in diameter and retains its defining bank of earth and stone in fair condition, though the northern to eastern arc has been partly obscured where a later field wall was built directly over it. That kind of quiet overwriting is common across rural Ireland, where generations of farmers repurposed ancient boundaries without necessarily knowing, or caring, what lay beneath.

Raths were the most typical form of enclosed settlement in early medieval Ireland, broadly from the sixth to the twelfth century, though many were built earlier and some continued in use later. They functioned primarily as farmsteads, the enclosing bank offering a degree of protection for livestock and defining a household's territory. What makes this example at Ballymullen particularly interesting is the presence of a souterrain within the interior. A souterrain is an underground passage or chamber, usually constructed from stone, that was built as an adjunct to such settlements. Their precise purpose is still debated, but they are generally thought to have served as cool storage spaces, as refuges, or perhaps both. Their construction required considerable effort, which suggests they were considered worth the trouble by the communities who built them.