But the 1874 structure it replaced was itself a replacement, and what came before that has vanished entirely, leaving only maps and a few lines of seventeenth-century prose to suggest that this quiet crossing in County Westmeath was once described as "the cheife pass into this country."

The Down Survey barony maps of 1654 to 1659, produced during the Cromwellian land settlement, depict a medieval bridge at Ballinalack carrying travellers along the routeway between Mullingar and Longford. The Down Survey was a vast mapping project commissioned to facilitate the redistribution of Irish land, and its maps are among the earliest cartographic records of many Irish sites. At Ballinalack, the crossing was considered significant enough to be guarded by two castles on the near bank, both recorded on the survey's map of Leny parish. In 1682, Sir Henry Piers of Tristernagh described the bridge as an "antient and well built bridge, which having of late been very incommodious and dangerous for travellers, is now very well repaired, adorned and rendered safe for them at the charge of the county." By 1837, Samuel Lewis recorded a five-arch bridge on the Mullingar to Longford road. That structure was replaced in 1874 by the Upper Inny Drainage Board, a body established to manage the improvement of the river's course, under the direction of engineer James Dillon. The river was considerably wider at this point before drainage works narrowed it, which explains why the earlier bridge needed five arches where two now suffice.

The 1874 bridge still stands and is worth a close look. Built from coursed squared limestone rubble, its arches are framed with rusticated rock-faced voussoirs, the wedge-shaped stones that lock an arch together, and a V-profile cutwater projects from the central pier to divide the current. A date-stone on the north face of the south parapet wall preserves the full inscription crediting the Upper Inny Drainage Board and naming Dillon as engineer. Nothing above ground marks the medieval crossing that once made this spot a matter of strategic importance.