A crannog, as these sites are known, is an artificial or partly artificial island, typically constructed from layers of timber, peat, brushwood, and stone, and built out into the shallows of a lake or bog. Crannogs were used as defended homesteads across Ireland and Scotland from the Bronze Age well into the early medieval period, and in some cases even later. The isolation they offered was the point: water made a more reliable barrier than a wall.

The Tully crannog is recorded as a monument, placing it within a long tradition of lake-dwelling that once dotted the Mayo landscape. The county's many loughs made it particularly suited to this kind of settlement, and surviving crannogs in the west of Ireland range from low, reedy humps barely visible above the waterline to more substantial platforms with traces of structural remains. Without further detail in the record, the specific history of this site, its date of construction, who built it, and how long it was occupied, remains to be established.