It measures twelve metres in length, sixteen metres in width, and rises to a height of 1.8 metres, with an opening of 2.7 metres facing west. That opening, and the stream running to the south, are not incidental details; they are the whole point.

A fulacht fia is a Bronze Age cooking site, found in enormous numbers across Ireland and Britain, typically dating from around 1500 BC onwards. The standard interpretation is that the mound represents the accumulated waste of a simple but effective method: a trough, usually dug near a water source and sometimes lined with wood or stone, was filled with water, and stones heated in a fire were dropped in to bring it to a boil. Meat could then be cooked in the heated water. Over repeated use, the stones would shatter from thermal shock and become useless, and the broken, burnt fragments were thrown aside, gradually building up the characteristic horseshoe-shaped mound around the old trough. The opening in the mound, as seen at Knockraheen, typically marks where the trough once sat, with the stream nearby providing a ready supply of water. Ireland has more recorded fulachtaí fia than almost anywhere else in Europe, yet individually they are among the least visited and least discussed of all prehistoric monument types, perhaps because a low mound of dark rubble in a field does not announce itself the way a standing stone or a ringfort does. The one at Knockraheen, quietly occupying its pasture beside a southern stream, fits that pattern exactly.