There is a long sandy beach at the back of the ferryport and you could have a nice walk there, weather permitting.
There is a steep drop down to the ferryport but if you walk up the hill and a little bit further, there is a shop and a restaurant.
Brilliant idea, its the sort of thing we used to do. Crazy but fun.
If you're coming as foot passengers there is no need to book far in advance so you can pick a nice day weather and sea wise.
You would be surprised how quickly 3 hours will pass. By the time you get off the ferry, walk to the terminal, walk from there up very steep steps to the top of the hill by Hotel Rosslare (wonderful view), take the cliff walk to a small kiddie's playground, or go to the beach, it will be time to return to the ferry. Not forgetting a cup of tea and something to eat, or ice-cream.