Customs returns from 1277 to 1280 record New Ross as the busiest port in the country, a fact that sits oddly with how quietly the place tends to feature in popular accounts of medieval Irish towns. Its Latin name offers a clue to its origins: Ros Pontis villa nova, meaning the new town of the bridge of Ros, a name that acknowledged both its novelty and its dependence on a river crossing that would be built, destroyed, and rebuilt across the centuries.

The town was founded by William Marshal, the great Anglo-Norman magnate, probably before around 1200, when the first of at least five successive bridges was thrown across the River Barrow. It grew quickly into the principal port for the Marshal lands across south Leinster, drawing trade from most of counties Wexford and Kilkenny and beyond. A poem from around 1265 describes the building of the town walls, though this account likely refers only to the digging of the fosse, the defensive ditch that preceded the masonry. A charter of 1374 suggests the town was still unwalled at that date, and murage, a tax levied specifically to fund wall construction, was collected most intensively between 1374 and 1420, with collection continuing as late as 1830. When walls were eventually completed, they enclosed roughly 105 acres, running from the river northward up Goat Lane to the Maiden Gate, along Nunnery Lane to a mural tower, and down William Street toward the quays. Several gates punctuated the circuit, including the Three Bullet Gate at the junction of Neville Street and William Street, and the Maiden's Gate toward the Irishtown, part of which still survives. Across the river, Rosbercon functioned as a separate, independent borough on the Kilkenny side and was probably never walled at all. The town's fortunes fluctuated sharply: by 1469 it had declined so far that the Kavanaghs were able to sack it, and in 1649 it surrendered to Cromwell without resistance, after which the fortifications were dismantled.

The density of religious foundations within and around the town is striking. Inside the walls alone there were five church sites, including Franciscan and Augustinian friaries, while Rosbercon held a Dominican friary and its own parish church. Outside the town lay the site of St Stephen's Church and, in an area called the Maudlins, what may have been a leper hospital attached to a monastic foundation. The Maudlins toponym, derived from Mary Magdalene, is commonly associated with such institutions across medieval Ireland and Britain. Trinity Hospital was established within the town in the sixteenth century. St Mary's retains considerable remains and is the most visible of the group today, though the broader pattern of foundations speaks to a town that was, for a century or so, a place of genuine consequence.