Oh where do you start! I'm not sure about Rosslare Harbour but if you sail into Dublin most people do an overnighter at Camac Valley, just outside Dublin. You can use it as a base to explore Dublin or move on.
Highly regarded campsites are Nagles Campsite, Doolin or Eagle Point Campsite, near Bantry in Cork, or Mannix Point which is on the ring of Kerry. Or go up to Donegal to Sleepy Hollows, Crolly. The Apple, near Cashel Rock in Tipperary is supposed to be very good but I've not been there yet.
I use Irish Ferries from Pembroke Dock to Rosslare,It is a shorter drive from Essex,but a slightly longer sea trip.
Once at Rosslare you can easily get to Dublin or Cork Kerry etc.I believe there is a motorway from New Ross that is open now.
Hope this is of help
John