Dungarvan Castle, also called King John's Castle, stands by the harbour on the site of an earlier, possibly Hiberno-Norse, fort. It was founded by Prince John, later King John of England, and its polygonal shell keep dates from around 1209. Shell keeps are unusual in Ireland: rather than a massive stone tower, the Normans built a high but relatively light outer wall with timber buildings inside it.
The castle is a National Monument, open daily from May to September.