Margaritaville is in Citywalk, which is inside the Universal resort, but outside of IoA. You don't need an IoA ticket to visit the restaurant. It's free to get in to. (Aside from parking).
Multi-day passes include CityWalk club access but you only need it for certain clubs...Margaritaville isn't one of them. If you have a one park ticket, you will not have all club access once the cover charges start to kick in so keep that in mind.
Maybe you can clarify something for me. Starting planning at full speed (for 9/12 trip) and then it fizzled out. My daughter wants to go sing at Rising Star and I believe there is a cover charge. If we have the 4 day park hopper, will this cover the charge to get in. This is something I have no clue about (CityWalk & Clubs). Thanks!