This strategic crossing point was chosen by Walter de Lacy in 1221 as the location for a new castle, though his ambitious plans were quickly thwarted. Cathal Croibdhearg Ó Conchobhair, King of Connacht, crossed the Shannon with his forces and compelled de Lacy to abandon the construction before it could be completed. Whether de Lacy’s original structure was intended to be built of earth and timber or stone remains unknown.

Six years later, in 1227, a castle was finally completed at this location, likely building upon or replacing de Lacy’s earlier efforts. The successful construction of this fortification appears to have catalysed the development of a settlement, which had already been granted a charter before 1235. The castle’s life was relatively brief; it was destroyed in 1271, after which it disappears from historical records. Its exact location within Lanesborough remains uncertain, though the most probable site is near the bridge where the Ordnance Survey maps of 1911-12 marked ‘Castle (Site of)’. This area, now a grass field behind St. Mary’s parish hall and adjacent to the town car park, likely saw further military use centuries later with the construction of another fortification in the late 16th or early 17th century.

The castle’s occasional appearance in historical documents under the name Ballyleague has led to some debate about whether it might actually have stood on the western, Roscommon side of the river, possibly at the location of a later tower house. However, no visible remains of such an early structure exist at that site, making the Lanesborough location the more credible option for this lost medieval stronghold.