The structure is a corbelled drystone oratory, a type of building in which courses of flat stones are laid so that each layer projects slightly inward over the one below, eventually closing to form a self-supporting roof without mortar or timber. The walls at Illaunloughan vary from 1.3 to 1.6 metres thick at the base, and when excavated they were found to stand to a maximum height of 1.29 metres, still clearly showing the corbelled and battered profile. No stone was dressed or worked. The builders simply selected flat pieces of Devonian sandstone, prised from a quarry ridge visible elsewhere on the island, and set them closely enough that the whole surface read as nearly level.

Excavations carried out over four seasons between 1992 and 1995 uncovered a building history of surprising depth for such a small site. Beneath the stone oratory, archaeologists found two earlier sod-built structures, possibly also oratories, and a leacht, a low stone memorial cairn associated with early Christian devotional practice, on the northern side. A radiocarbon date of 640 to 790 AD was obtained from a layer overlying the clay floor of the stone phase. The unusual step down from the threshold into the interior appears to be a consequence of constructing the stone building directly over one of the earlier sod structures, whose sill stone remained in place at the lower level. The entrance jambs are splayed, narrowing from 0.95 metres on the outside to 0.62 metres within, but lack the vertical facing uprights seen at the comparable oratory on Skellig Michael. No window survives, though a single east-facing window was conventional in such buildings elsewhere in Ireland. The island's name adds another layer of uncertainty: it may preserve the name of a saint called Lochán, two of whom appear in the Martyrology of Oengus written around AD 800, or it may simply mean the island of the chaff, as one older source suggests. There is no documentary record to settle the question either way.