It’s not a journey one would want to make in one day. I recomend staying in Cork (somhow) overnight. Maybe do the East Cork lines too.
belfast-Connolly has first class (unless it breaks down as my Cork-Belfast leg did, great fun sitting the 2nd half of a 6 hour journey in a 29k!), but, thanks to the ongoing refurbishment of the mark 4s. The Heuston-Cork leg of your journey will either be in the old City Gold, Or the New 1st class and it’s impossible to tell which trains have the new and improved first class, versus the old and a bit tired city gold.
just yesterday I was in Kent following the st Patrick’s day special trains on the east Cork lines and counted of the 3 mark 4 sets I saw, 2 of them still had city gold