My understanding is that there will be up to 6 additional services each way between Clonmel and Waterford. Something similar between Kulkenny and Waterford. Last trains to both locations after 2300 on each of the 3 days of the festival.
If they added a 21:15, 22:15 and 23:15 service to kilkenny. It's possible to do it.
There needs to be more services to Dublin on the Sunday. Most people will come down on Friday and go back on Sunday.