It measures roughly fifteen and a half metres north to south and ten metres east to west, rising just over a metre above the surrounding pasture. The careful orientation is not coincidental. This is a fulacht fia, and the wet ground beside it was the whole point.

Fulachtaí fia are among the most common prehistoric monument types in Ireland, found in their thousands, particularly in Munster. The name is sometimes translated as "deer roast" or "wild animal cooking place", though their precise function has been debated. The standard interpretation is that they served as cooking sites: stones were heated in a fire, then dropped into a water-filled trough to bring it rapidly to the boil. The shattered, fire-cracked stones, discarded after use, gradually accumulated into the characteristic horseshoe mound that survives today. At Derrymihin, those heat-shattered stones and charcoal-enriched soil are still clearly visible at the tip of the southern arm, exposed as though waiting for analysis. A row of upright stones appears to form a revetment, a retaining structure, along the inner curve where the arms of the mound meet the opening, suggesting some deliberate effort to hold the shape of the site together. Most fulachtaí fia date to the Bronze Age, broadly between 1500 and 500 BC, though some have produced earlier or later dates.

The site sits in rough pasture and is largely unexcavated. The gorse that covers much of the mound makes its profile a little harder to read at close range, but the scale of the monument becomes clear when you trace the curve of the arms and register just how much broken stone had to accumulate here over repeated use to produce a mound of this size.