There is no problem with Rosslare Europort being the terminus, but Wexford-Waterford has to have priority with the peak flows and have the generally better service.
So it could operate:
Rosslare Europort-Wexford-Waterford
Waterford-Wexford-Rosslare Europort
Felthouse-Killinick as a direct curve would only save 5km between Wexford and Waterford, but you would need to build 3km of railway, money that would be better spent elsewhere. It ran from here:
http://maps.osi.ie/publicviewer/#V1,706971,616107,6,3 to here:
http://maps.osi.ie/publicviewer/#V1,705249,613363,6,3