Illaunyregan, like many of the minor islands scattered along Ireland's western seaboard, holds an enclosure, the kind of feature that archaeologists classify carefully but that resists easy explanation. Enclosures of this type can range from the remains of an early medieval farmstead to the boundary wall of a promontory fort or a monastic precinct, their original purpose often legible only from the shape of the land and whatever survives above ground.

The island's name itself is worth pausing over. Illaun derives from the Irish oileán, meaning island, and the second element, Yregan, likely preserves a personal name or an older territorial designation, the kind of compounded place-name that Irish cartographers and scribes recorded over centuries without always explaining. That a defined enclosure survives here at all suggests some deliberate human activity, a community, a boundary, a defended space, at some point in the past. The detail of what precisely was enclosed, by whom, and when, remains to be fully examined.