I haven’t used Rosslare since Irish Ferries transferred their France route to Dublin, which is several years ago now. I did on at least one occasion overnight in the ferry terminal car park without issue, but long time back.
I looked on search for sites and am disgusted at the number of nearby locations now having height barriers. Welcome to Ireland - not!
The location that I parked at was (in what3words code) streams.highways.jubilation . Might be worth a call to the terminal to confirm this is still acceptable.
Davy
I looked on another app and notice that someone was able to park in the ferry terminal carpark for €5 using a car parking app to pay. This was at the start of the year, so looks possible.