A narrow grassy saddle, incorporating a massive chunk of rock, still connects it to the mainland cliff, and a precipitous path descends from the landward side to this crossing point. Beyond it, a steep grassy slope climbs to a summit more than ten metres above the sea. That summit, irregular in plan and roughly 43 metres along its length, is the remains of a promontory fort, a class of defensive enclosure in which a headland or coastal projection was sealed off from the land by one or more earthen or stone banks, turning the sea itself into a natural perimeter wall on all other sides.

The site appears as 'Doonallia Isd.' on the first edition of the Ordnance Survey map of 1840, and scholars including Mac Neill in 1913 and Ó Muraíle in 1999 have noted the place name. When the antiquary Thomas Westropp visited and wrote about the site in the early twentieth century, the saddle connecting the promontory to the mainland was a substantial feature: in his 1914 account he described a lofty causeway of earth and stones, faced on either side with drystone walling, 1.8 metres wide and often reaching 3.65 metres in height. That structure has since been largely eaten away by erosion, collapsed into the boulder beach below, though the neck of ground it once crowned remains dry even at spring tides. The defensive banks that once enclosed the summit have fared no better. Westropp recorded a slight bank of earth and stones around the landward edge, but today only two small grassy mounds survive, each less than a metre high, perched close to the cliff edge at the south-western, highest end of the summit. A possible fragment of bank also survives along the western perimeter, measuring roughly two metres in length and just thirty centimetres high. The undulations of four associated house sites are still readable underfoot beneath the grass and summer ferns.

The approach to Doonallia is genuinely awkward. The path down to the saddle is described as very narrow and precipitous, and the grassy slope beyond rises steeply to the summit. In summer the upper ground becomes overgrown with ferns, which can obscure the subtle earthwork remains. A second possible promontory fort lies approximately fifty metres to the south-east, suggesting that this exposed corner of Clare Island's coast held some sustained significance, though what precisely drew people to fortify these wind-scoured stacks is a question the eroding ground is slowly making harder to answer.