IRC is required for both classes. It's has cost around $275 in previous years. Don't forget you'll need a USAC membership, which is currently $85 for a single race.
As to course specifics? No one knows until the week of the race. Odds are that the sportsman will be bypassing most of the harder rock trails, but using the same pits and same desert section. My understanding is that the premise of the Sportsman class is so that the 'average' UTV owner will show up and get their feet wet, without having to spend as much money as the pros, and without having to worry about vehicle damage.
If you want to run the rocks, best bet is to race in the Pro Class. Plus, you get a chance to win money!