Ballylahan, derived from the Irish 'Baile-Atha-lethain', meaning roughly 'Town of the Broad Ford', once denoted a genuine market settlement on the River Moy. No trace of it survives today, and its exact location, extent, and character remain unknown. The ford itself is gone from view, the town is gone, and what remains is a townland name carrying the outline of something that was once, by definition, a place where people gathered to trade.

The settlement grew up around Ballylahan Castle, an Anglo-Norman fortification raised in the mid-to-late thirteenth century by a man named Jordan de Exeter. The castle was positioned deliberately at a fording point on the River Moy, since controlling a river crossing in medieval Ireland meant controlling movement, trade, and access across a landscape where roads were few and waterways were arteries. Anglo-Norman lords frequently planted market towns beside their strongholds, the settlement and the fortification reinforcing each other; the castle offered protection, while the market drew people and revenue. Ballylahan appears to have followed this pattern, though how long it functioned, how large it grew, and precisely when it was abandoned are questions the ground can no longer answer.