If they are CAMRA's rules, and it's CAMRA's festival, then tough. If you don't like it, join CAMRA and get it changed or run your own festival.
Cask beer will not become a regular feature on the Irish market because it is too expensive for publicans to stock in terms of the environmental conditions it requires and the potential for wastage. Having it on tap in Belfast for a few hours each year will not change this.