Locals know the spot as a fort, and that name carries more weight than the modest earthworks might suggest. What has largely vanished from view is what once lay beneath it: an underground passage, the defining feature of a souterrain. Souterrains are stone-lined tunnels or chambers built underground, most commonly associated with early medieval settlement in Ireland, and thought to have served variously as storage spaces, refuges, or escape routes. The one at Lack has left only its surface traces behind.

The remains today are, in the words of the 1986 Dingle Peninsula archaeological survey compiled by J. Cuppage, confused and difficult to interpret. What survives is a roughly semi-circular stony mound, no more than a metre high and around ten metres at its longest, its north-western edge defined by a field wall that has become part of the structure almost by accident of proximity. Directly to the east of the mound, a low stony bank runs north to south for approximately 14.5 metres, standing about 0.6 metres high and three metres wide. Whether this bank is a remnant of an enclosing feature, a later field boundary, or something else entirely is not clear. The underground passage itself no longer appears to be accessible or visible, leaving the site caught between what it once was and what the landscape has made of it since.