There's not enough space for that arrangement in the current complex, but the land alongside it is largely agricultural and shouldn't cost too much to acquire.
You would have to double-track the line through Limerick Junction at least and then construct two platforms and link them to the main platforms.
There is again this problem of stopping trains before crossing the Dublin-Cork mainline, so you would probably have to stop Limerick trains north of the junction and Waterford trains south of it. The Limerick platform would be west of the line and Waterford platform east of it.
Then multiple bridges are required for platform access.
It gets complicated.