The town was founded in the 13th century on an important crossing point by an Anglo-Norman mercenary from Wales,
Thomas FitzAnthony, replacing the earlier Irish settlement of Grennan (, Sunny Place). It is from FitzAnthony that Thomastown was named. FitzAnthony was granted a large area of land in the region by
William Earl Marshall, son-in-law of
Strongbow, and became the Seneschal (Governor) of Leinster in the 13th century. He built fortifications at Thomastown, fragments of which can still be seen today, together with nearby Grennan Castle, now in ruins. FitzAnthony died in 1229. Of this castle and the town's walls, the only remains are the towers near each end of the bridge and the remains of a 13th-century church, dedicated to St Mary. The town thrived and had more than 200 burgesses at the end of the 13th century. Thomastown became a small medieval walled town: the first walls were built in 1449. Edward III (twice), Henry VI, Mary I, James I, and James II granted the town royal charters. Under a 1553 charter from Queen Mary I, the burgesses of Thomastown had the right to choose two members of Parliament, a practice they continued until the
Acts of Union in 1800. In 1650 the town was attacked by
Oliver Cromwell. Grennan Castle was laid siege to by Cromwell's army and after two days the defending forces surrendered. Local tradition holds that the remains of
Saint Nicholas, the 3rd century
Anatolian
bishop, lie in the vicinity of Thomastown in Jerpoint Park. A grave slab with the carved heads of three people at the ruined Church of St. Nicholas, the church itself, and other stones are virtually all that remain of the medieval village of Newtown Jerpoint, which had fallen into ruin by the 17th century. The village of Newtown was adjacent to Jerpoint Abbey, founded in 1183. The abbey had its own gardens, watermills, cemetery, granary, and kitchens, and was home to a group of Irish-Norman Crusaders in the Middle Ages. It was dissolved in 1540. The legend refers to a band of Irish-Norman knights from Jerpoint, who travelled to the Holy Land to take part in the Crusades. On their return to County Kilkenny, it is said they brought St. Nicholas' remains. ==Landmarks==