That phrase, indeterminate date, carries a particular weight in archaeological classification. It means the structure could not be assigned to any recognised period with confidence, leaving it suspended somewhere between the deep past and the recent, a building that resists easy categorisation.

Achill has a long and layered human presence, from megalithic tombs on its upland slopes to the booley villages used by seasonal herders well into the nineteenth century. Keel itself sits near the southern shore of the island, facing Keel Lough and the broad strand beyond. Without more detail about the structure's form, its materials, or its relationship to surrounding features, it is difficult to say whether it belongs to the tradition of vernacular cottages common across west Mayo, to an earlier agricultural or pastoral way of life, or to something else entirely. The designation simply records that something is there, and that its age remains unresolved.