What makes the site quietly arresting is not what survives but what is disappearing: much of the interior has already slipped into the sea, and the traces that remain are subtle enough that they could easily be mistaken for natural features of the coastal landscape.

A promontory fort is, in essence, a defensive enclosure that uses the natural drama of a headland to its advantage, relying on cliffs or steep drops on most sides and concentrating any man-made defences across the narrow landward neck. At Gleann Lasra, the headland faces north-west and its neck is particularly narrow. A natural crevice running across that neck appears to have been deliberately deepened into a fosse, a defensive ditch, and roughly 40 metres to the south lie the collapsed remains of what was once a revetted wall, meaning a wall built against an earthen bank to hold it in place. The interior, now largely eroded away, gives no further clues about the people who once used it. The site sits within a cluster of three promontory forts along the same stretch of coastline, with a larger and better-preserved example at Conaghra visible to the north, making this part of Mayo an unusually concentrated area of this type of monument. The site was brought to attention through fieldwork underpinning the 1999 MA thesis of Markus Casey, a systematic survey of coastal promontory forts across Sligo, Mayo, Galway, and Clare.