No date, no builder, no surviving description of what it once crossed or connected. A causeway, by its nature, implies two things that needed joining and something, water or bog or difficult ground, that stood between them. Whatever that obstacle was at Lannagh, and whoever decided it was worth the effort to overcome it, remains for now unrecorded in any publicly available form.

The townland of Lannagh lies in the west of Mayo, a county whose landscape has been shaped over millennia by blanket bog, river systems, and the kind of low, waterlogged terrain that made causeways genuinely necessary rather than merely convenient. Historically, causeways in Ireland ranged from simple arrangements of laid stone across shallow water to more substantial engineered crossings, some of medieval origin, others reaching back into prehistory. Without the supporting detail for this particular structure, it is not possible to say which tradition it belongs to, or whether it survives intact, partially, or only as a trace in the ground.