The rath at Ballymarcahaun measures 35.5 metres in diameter, a modest but coherent ring of raised ground that has survived centuries of agricultural activity in broadly fair condition. What lifts it beyond a simple earthen circle is what lies beneath: a souterrain within the western half of the interior, an underground stone-lined passage or chamber of the kind commonly associated with early medieval settlement in Ireland.

Raths, also called ringforts, were the most common form of rural settlement in early medieval Ireland, typically enclosing a farmstead and its associated structures within a bank and ditch. They were built and occupied roughly between the sixth and twelfth centuries, though many continued to shape the landscape long after that, accumulating folklore and, in some cases, concealed subterranean features. The souterrain, a type of underground passage usually constructed from drystone walling and roofed with large slabs, is thought to have served various purposes depending on the site, including storage, refuge, and ventilation for perishable goods. Its presence here, tucked into the western portion of the interior, suggests this was once a functioning domestic site rather than a purely symbolic or defensive enclosure.