There are some ruins of a castle in the middle of town. To gain access - which is free - you turn right (assuming you are facing the main gate of the castle) and walk down the street to the key keeper's house. (Directions are on the gate of the castle). This man is possibly one of the most charming you can meet in Ireland - ask him about the history of the area, and he will tell you stories of battles, including the one that destroyed that castle, as if he was there. Take a tape recorder, his stories are a treasure. And if you appear to be a responsible adult, he'll hand you the keys, tell you to be safe exploring and to lock up when you're done, and you can wander the ruins on your own. There really isn't much, but there is a tower to climb, and a wonderful view at at the top. Ferns itself is bearly a blip on the map, but it includes the castle ruins, wonderful abby ruins, and a graveyard that contains the grave of a son of Brian Boru. Brian Boru was the first chieftain to unite the tribes of Ireland, and his sons were the first "O'Briens". So if you are related to the O'Briens, this may be the grave of one of your ancestors.