It is, in fact, a rath, the Irish term for a ringfort, and its low profile belies a considerable age. At 1.3 metres high, the bank is still largely intact, its outer face retaining traces of the stone that once gave it a more defined, structural character.

Ringforts were the most common form of rural settlement in early medieval Ireland, built and occupied roughly between the fifth and twelfth centuries. A rath specifically refers to a ringfort whose enclosing boundary is made from earth, sometimes revetted or faced with stone as this one appears to have been, distinguishing it from a cashel, which is built entirely of dry stone. They typically enclosed a farmstead, protecting a family, their livestock, and their stores from opportunistic raids rather than organised military attack. Thousands survive across Ireland in varying states of preservation, though many have been lost to agriculture and development over the centuries. The one at Derryduff sits in level pasture, which may partly explain its survival; low-lying, open ground offers less reason to clear a feature that, overgrown as it is, does not obstruct farming in any dramatic way.