Unfortunately, the container terminal in Cork is moving from Tivoli to Ringaskiddy and will not be rail connected (of course the plan is to build a new motorway there right through the middle of a suburban area, but road funding is how things work in this country). There's no great prospect of a rail connection either as the options would be very expensive, with either a new rail line right through the suburbs of Cork (major CPOs so expensive) or an underwater tunnel from the Cobh line so incredibly expensive from an engineering perspective).
For me, Belview is the obvious connection point if there is to be a rail/port connection on the south coast. It can handle containers already (unlike Rosslare) and is properly rail connected (unlike Cork).
I doubt freight makes the case for the WRC though. There's no great benefit in shipping rail freight fast as we don't move time-sensitive goods over the rail network (and this is the case in countries with much more elaborate rail freight). While there is a case for freight trains avoiding the Dublin area because of disruption to existing services, there's no major issue with going Ballina-Athlone-Athenry-Limerick-Belview on the current network, with no capital investment required. If the WRC is reopened, it will need to stand on projected passenger numbers.