Before 1664 this place was known in Irish as Béal Átha Liag, meaning the mouth of the ford of the flagstones, and the crossing it names was no minor local convenience. It was a major fording point on the Shannon, and more significantly, it sat directly on the boundary between the early medieval kingdoms of Connacht and Meath. Control of that crossing was, in practical terms, control of a frontier.

The strategic weight of the place drew the attention of powerful figures. Around AD 1000, Mael Sechnaill, King of Midhe, constructed a causeway here, and a separate causeway was also built by Cathal Ua Conchobhair, though the annals do not fix precise dates for both works. A causeway in this context would have been a raised, reinforced trackway across marshy or shallow ground, designed to make the crossing passable in all seasons and, crucially, to assert a kind of territorial ownership over the route. The exact locations of these structures are no longer known. They were probably superseded by later bridges, and the modern town grew up around a crossing point whose ancient significance had long since been absorbed and forgotten beneath subsequent layers of construction and settlement. What had once been Béal Átha Liag became Lanesborough, an anglicised name dating from the mid-seventeenth century, and the flagstones that gave the ford its Irish name left no trace in the record that survives.