It is classified as a promontory fort, a category of prehistoric enclosure that typically used natural cliff edges and man-made ramparts to defend a headland or isolated outcrop. The trouble with Doontraneen Island is that nobody has ever been able to confirm that any fortification existed there at all.

The island's name offers a clue to its intended purpose. The scholar Mac Neill, writing in 1913, rendered it as 'Dún Tràgha', meaning roughly 'fort of the strand or ebb', a reference to the tidal conditions that govern access to it. Its base can be reached on foot only at very low water, which would have made it naturally defensible without much additional construction. The antiquarian Thomas Johnson Westropp described the site in 1911, recording a memorable detail: a certain Mr Praeger swam out to the stack and hauled himself to the summit, only to find no structural remains whatsoever. Westropp did not dismiss the fort classification on that basis, however. He speculated that the soil cap of the summit had slipped away over the centuries, taking with it whatever slight fences or hut-sites may once have stood there. It is a reasonable hypothesis for a site this exposed, and one that cannot easily be disproved. The island appeared by name on the first edition Ordnance Survey map of 1841, and the third edition of 1920 recorded its area as two roods, a traditional land measure equivalent to roughly half an acre, which gives some sense of just how small this place is.

The sloping grassy summit remains bare of any visible archaeology today. What persists is the outline of a place that was almost certainly considered significant at some point, given a name that encodes its defensive logic, mapped and measured by surveyors, swum to by at least one determined naturalist, and still sitting in the tidal zone off Mayo, accessible only briefly and on the sea's own terms.