Waterford is Ireland's oldest city, founded by Vikings in 914 AD, and it still carries that long history in its medieval street pattern, surviving city walls, and the sturdy 12th-century Reginald's Tower at the waterfront. The city sits on the River Suir with some of the most scenic river landscapes in the southeast, and until the 17th century it was Ireland's second city after Dublin. It retains the best-preserved city walls in Ireland after Derry.
Waterford is best known internationally for Waterford Crystal, and a visit to the factory (weekdays only) to watch crystal being made from start to finish is worth half a day. The medieval centre around Reginald's Tower is compact and walkable, with fine Georgian houses and narrow alleyways. The French Church, where Huguenots once worshipped, and the Cathedral are both worth a stop.
In September the Waterford Light Opera Festival brings the town to life. Waterford is generally treated as a brief stop between Cork and Dublin, but it deserves at least a night — use it as a base to explore the surrounding countryside.