For heaven's sake, this discussion is a bit ridiculous, considering that Irish Rail cannot even manage to solve some very simple constraints on congested lines (like upgrading the through road platform at Enfield to take passengers).
A platform on the down main line at Lim Jcn may make life easier as far as conflicts between up and down Dublin-Cork trains are concerned, but connections from down trains will no longer be cross platform but would involve climbing/descending stairs/lifts to an overbridge/underpass across three lines. What does that do for connecting times, which at present can be very slick.
This thread is meant to be about the Limerick-Junction to Waterford line, and what is really required is some investment to lower the journey time from 1h40m to about 1h20m. This is mainly a question of better signalling and visibility at crossings.