It is a curious inversion: the most historically significant fabric is not the town centre's commercial frontage but a modest lane, its remains pointing back to a settlement that was already described as a going concern in the mid-seventeenth century.

Ballymahon grew up on the northern bank of the River Inny, at a point where the river could be crossed, and that crossing shaped everything about the place. A land grant in 1620 to James Mc William Ferrall named the castle, town, and lands of Ballymahon, confirming that some form of organised settlement existed before the upheavals of that century took hold. The Down Survey map of Rathcline barony, drawn between 1655 and 1656, shows a cluster of houses on the northern bank, sitting to the east of a bridge. By 1682, a contemporary account described Ballymahon as a market town with a wooden bridge over the river. The Civil Survey, a mid-seventeenth-century administrative record of landholding and settlement across Ireland, paints a more fragmented picture: it notes a mill alongside the ruins of a castle, along with what it calls "chimney houses and Irish creats," the latter being a term for temporary or makeshift dwellings associated with a more mobile, pastoral way of life. The castle itself is gone, and when archaeologists carried out test excavations in 2005 ahead of a housing development behind the south-west side of Main Street, they found no archaeological features or artefacts at all, suggesting the physical evidence of those early layers has either vanished or lies elsewhere in the town's fabric.