The modern settlement is, in all likelihood, a relatively recent development overlying the ghost of a medieval borough that was abandoned centuries before the current streets were laid out. That older place had a name of its own: Natherlagh, the Irish name for Aherlow, and it functioned as a functioning Anglo-Norman urban settlement with burgages and burgesses, the plots and tenants that formally defined a medieval borough under English colonial law.

The de Moulton family were the Anglo-Norman lords associated with the foundation of this borough, though precisely when and how they established it remains unclear. The manor sat outside the main grants of land distributed under the formal colonisation of Limerick and Tipperary, which may explain why the documentary record is thin. What survives is an extent, essentially a formal survey of a lord's landholdings and their value, made in 1341, the same year the manor passed from the de Moultons to the de Birminghams. That document records both burgages and burgesses at Natherlagh, confirming that a borough existed in some organised form. The borough's position on the frontier between the Anglo-Norman colony and the territories of the native Irish proved to be its undoing. By the later fourteenth or early fifteenth century, the Gaelic Revival, the broad reassertion of Gaelic Irish political and military power across much of Ireland, had almost certainly caused the settlement to be abandoned. The area around the castle and parish church is considered the most probable location of this lost medieval core.

Visitors approaching Galbally today will find a quiet rural town whose medieval layers require some imagination to read. The castle and parish church referenced in the Urban Survey of Limerick are the most tangible pointers to where Natherlagh once stood, and locating them on the ground gives a rough sense of the earlier settlement's footprint. The surrounding landscape, the Glen of Aherlow opening out behind the town, adds geographical logic to the site: this was always a boundary place, positioned where different territories met, which made it both useful and, ultimately, vulnerable.