What emerged was a low mound of very dark earth mixed with burnt stone, the characteristic signature of a fulacht fia, a type of ancient outdoor cooking place found across Ireland in their thousands. The typical interpretation is that these sites were used to heat water by dropping fire-cracked stones into a trough, bringing it to a boil for cooking meat. Over repeated use, the shattered, blackened stones were discarded in a heap beside the trough, and it is that accumulated mound which survives into the present.

The boggy, level terrain at Glasnamullen is entirely typical of where fulachtaí fia tend to cluster. Wet ground made it easy to dig a trough that would hold water, and marshy areas also preserved the organic and charred material that makes these sites so legible to archaeologists. What gives the Glasnamullen site a particular interest is that it does not sit in isolation; two further fulachtaí fia have been recorded nearby, suggesting this stretch of low-lying Wicklow ground saw repeated or sustained use over time. Whether that reflects seasonal activity, a favoured landscape corridor, or simple coincidence of survival is difficult to say, but the concentration is notable.