It is a fulacht fia, a type of ancient cooking site found in enormous numbers across Ireland, typically dating to the Bronze Age. The mound here is roughly oval in plan, stretching eighteen metres east to west and eleven metres north to south, and rising to about 0.7 metres above the surrounding ground. That modest height is characteristic: these features rarely announce themselves dramatically, which is part of why so many survive unnoticed in farmland for millennia.

A fulacht fia generally consists of a horseshoe-shaped mound of shattered, fire-cracked stone, the debris left over from repeated heating. The standard interpretation is that stones were heated in a fire and then dropped into a water-filled trough to bring it to a boil, most likely for cooking meat. The process is reliable and surprisingly efficient, and experimental archaeology has confirmed it works well in practice. Over time, the thermally fractured stone accumulates into the characteristic spread that gives these sites their distinctive form. At Rockspring, the burnt material beneath the grass cover is what defines the mound, and the site was identified not only by ground survey but also as a soilmark visible in aerial photography, a reminder that some of Ireland's oldest features are best understood from above, where subtle variations in soil colour and crop growth reveal what the surface conceals.