It measures roughly ten metres north to south and six and a half metres east to west, rising only about twenty centimetres above the surrounding ground. Beneath the grass, it is composed of shattered stone fragments set in a dark, charcoal-rich soil, the unmistakable signature of a burnt mound, a class of prehistoric site found in considerable numbers across Ireland and Britain.

Burnt mounds, sometimes called fulachtaí fia in Irish, are among the most common yet least celebrated archaeological features in the Irish landscape. They are generally interpreted as the debris from repeated heating of stones in fire and then plunging them into water-filled troughs to bring the water to a boil, a process that fractures the stone and gradually builds up a characteristic spread of cracked, fire-reddened fragments in dark organic soil. What the boiling water was actually used for, whether cooking, bathing, textile processing, or something else entirely, has been a matter of lively archaeological debate for decades. At Ballyglass, there is a shallow, slightly sunken indentation near the northern end of the mound's eastern side, approximately two metres by one point eight metres, which may represent the ghost of one such trough hollow, though this is uncertain. The western edge of the mound is indistinct, merging gradually with the surrounding field, and the southern side is bordered by a partly silted-up, vegetation-clogged drain.

The mound does not sit in isolation. On the crest of the slope directly above it is a rath, a type of enclosed farmstead typically defined by an earthen bank and ditch, common throughout Ireland from the early medieval period. The relationship between the two features is not spelled out, but the juxtaposition of a prehistoric cooking or processing site at the slope's base and a later enclosed settlement on its summit is a reminder that landscapes in rural Mayo were being layered with human activity across many centuries. To the north, low-lying boggy pasture stretches away, the kind of wet, marginal ground that would have made reliable water supply for a burnt mound straightforward enough.