The port town of New Ross dates from pre-Middle Ages. The earliest settlement in this area dates to the 6th century when St. Abban of Magheranoidhe founded a monastery in what is now Irishtown. The original earthen banked circular enclosure of his monastery was visible around the graveyard until it was removed by the council. It was replaced by a concrete wall and steel fence. Its name, Ros, was shortened from Ros Mhic Treoin, or the Wood of the Son of Treoin.

New Ross was in the territory of Dermot McMurrough and came to prominence when the Anglo-Normans conquered the region. The Norman knight William Marshall and his bride Isabella de Clare arrived during the early part of the 13th century. An earthen defensive structure called a motte was built at Old Ross in order to hold the newly conquered territory. A medieval borough sprung up around it - peopled by English and Welsh settlers. The arrival of Isabella and William is described in the Chronicles of Ross, which are in the British Museum. It records that in 1189, Isabella set about "building a lovely city on the banks of the Barrow". The town's fortunes further increased when King John made William the Earl of Pembroke at his coronation in 1199. A year later, the Earl Marshal transferred the Norman capital of Leinster to Kilkenny and New Ross became the main port.

The town grew around the bridge built by William Marshal, son-in-law of Richard de Clare, 2nd Earl of Pembroke (Strongbow), and a leader of the Norman invasion of Ireland. The town of New Ross (the town of the new bridge) was granted a Royal Charter in 1207. The port gained concessions from King John in 1215 and again in 1227 but these were later revoked by Henry III and Edward I to protect the port of Waterford. New Ross was still Ireland's busiest port in the 13th century. These restrictions were lifted in the 14th century by Edward II and Edward III.

St Mary's Abbey (Church of Ireland) was built in 1811. There are two Roman Catholic churches, the parish church of SS. Michael and Mary completed in 1902, and the Augustinian church opened in 1835.

The town was fought over in the Irish Confederate Wars of the 1640s. In 1643, the town resisted the siege by James Butler, 1st Duke of Ormonde, who fought a battle near the town with an Irish army under Thomas Preston, 1st Viscount Tara but later re-conquered by Oliver Cromwell in 1649 who discharged three cannon shots at the Aldgate. The town is at an important crossing point, sited on the River Barrow between the river estuary to the south and the point where the River Nore joins the Barrow to the north. It was the location of one of the bloodiest battles of the 1798 rebellion.