The name itself offers a small clue to the character of the place: bilberries, the low-growing wild fruit common to boggy, acidic ground across the west of Ireland, tend to thrive in the kinds of exposed, undisturbed terrain that often surrounds island sites left largely to themselves for centuries. That a named island in this part of Mayo carries a monument designation at all suggests something was once built, buried, or otherwise shaped by human hands on or around it.

Mayo's lakes and wetlands are scattered with islands that served as places of refuge, ritual, or habitation across many centuries. Some are crannogs, artificial or partially artificial islands constructed during the early medieval period, built up from layers of timber, stone, peat, and brushwood to provide a defensible platform in the middle of water. Others are natural islands that accumulated traces of use over time, from seasonal grazing to the siting of a church or a chieftain's stronghold. Without more detail specific to this site, it is not possible to say with confidence which category Bilberry Island falls into, or what period its remains belong to. It is one of those places that has been noted, named, and classified, but not yet fully described in any accessible form.