Realise I'm 3 years late but all the infrastructure from Cherryville to Waterford was built to accomodate double track (and immediately north of Waterford there was room for a third track carrying Limerick Junction trains). Thankfully although most rail-over bridge decks are single track, the supports are all still wide enough to support double track the entire way, and new bridges over IÉ alignment are as wide as practical by default, so of course have room for a second track.
The only point at which there isn't room is the <200m section over Thomastown viaduct, but as demonstrated at Drogheda, a very short single-track section is manageable, especially adjacent to a station.
It would involve replacing the aforementioned majority of single-track rail-over spans, but all other masonry can accomodate dual track, and aforementioned triple track would also be possible north of Waterford, although I'm not sure it's necessary given that the Limerick Junction line would be unlikely to ever go above one train per hour per direction and the Dublin line is unlikely to go above 2 in the next few decades.