Its not a single issue party. They are radical right wing hyper-Christians who went to bible schools in South Carolina.They are very against gay marriage and abortion. Also they are an ethno-nationalist party and their ethnic group won't disappear overnight.
Once unification happens they will still have supporters in the North. Definitely older people but hopefully the young ones will move away from them. If they are smart they could drop the Unionism thing and be a all-Ireland far right party and pick up some votes in the 26 counties.