That the interior floor has been deliberately raised on its northern side, to counteract the natural tilt of the hillside and create a level living space within, is one of those small engineering details that tends to catch people off guard. It suggests not just effort, but considered planning on the part of whoever built it.

This is a rath, the Irish term for a ringfort enclosed by an earthen bank rather than stone walling, and it belongs to a type of monument that was built in large numbers across Ireland roughly between the sixth and tenth centuries. They served as enclosed farmsteads, the bank and its ditch providing security for people, livestock, and goods. The Reenroe example measures approximately 19.5 metres north to south and 18.4 metres east to west, making it a fairly modest example of the form. Its bank is faced with stone in places, which is a refinement not always seen, and there is a stepped scarp running from the north-east around toward the east-north-east, suggesting the outer face was shaped with some care. A narrow gap of about 1.2 metres in the eastern side of the bank marks where the original entrance would have been, the standard placement for a rath, broadly in keeping with where morning light would fall.