The site at Coolroe is a fulacht fia, a type of monument found in enormous numbers across Ireland, typically consisting of a horseshoe-shaped mound of fire-cracked stone built up around a water-filled trough. Prehistoric cooks, or possibly people engaged in other activities entirely, would heat stones in a fire and drop them into the trough until the water boiled; the discarded, shattered stones accumulated over time into the characteristic mound. The one at Coolroe was modest in height, around 0.7 metres, but spread to roughly 17 metres east to west and 12 metres north to south.

The site was fully excavated in 1998 by archaeologist R. Gillespie, ahead of construction work by Mayo County Council on a sewage treatment works. What the excavation revealed was more complex than the surface suggested. At the centre of the mound sat a large oblong trough measuring 3.5 metres by 2.3 metres, its sides reinforced by approximately 35 timber stakes, with a second, square trough nearby measuring 1.6 metres on each side. A post-hole and a drain were also recorded. Among the finds were struck chert flakes, fragments of antler, and worked wood, small but telling signs of human activity. Two further fulacht fia sites lay just a few metres to the west and east respectively, and an area of broader prehistoric activity was recorded around 80 metres to the west, suggesting the ground around the Robe River had drawn people back to the same spot across generations.