Nobody is entirely certain what it was. The structure measures roughly fourteen metres north to south and twelve and a half metres east to west, and somewhere inside it a second wall once ran from west-south-west to east-north-east, dividing the interior into at least two areas. That internal subdivision is the detail that makes the place quietly puzzling, because it suggests either a small dwelling with a partition wall or, perhaps more likely, a small enclosure of the kind that might have served any number of agricultural or ecclesiastical purposes.

The proximity to the ecclesiastical site next door is presumably not coincidental. Early Christian and medieval religious foundations in the west of Ireland were frequently surrounded by subsidiary structures, including enclosures for livestock, gardens, or the accommodation of monks and pilgrims, and the relationship between the two features here has not been firmly established. Drystone construction, in which stones are laid without mortar, was the default building technique across the Aran Islands and the Connemara coastline for centuries, and it leaves little that survives well once a structure falls into disuse. What remains at Cill Mhuirbhigh is described as poorly preserved, the walls much collapsed, which places any confident dating beyond reach. The structure appears in Paul Gosling's Archaeological Inventory of County Galway, published in 1993, listed simply as a house of indeterminate date, with the caveat that the enclosure interpretation may be the stronger one.