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). 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 became a small medieval walled town.