I know it may seem a herculean task, but you need to swallow the irritation. If you confront them while you're thinking the way you've written this post, it will show through. I can tell you from dealing with a younger brother that you need to master yourself before you can master the situation around you.
If they're being truly loud and obnoxious, some other patrons have probably noticed as well. Take some time to look around the room and see if you can spot the annoyed glances in their direction. If you approach others who are also annoyed at the boisterous behavior, they'll probably help you confront the source.
This adds some validity to your complaint, since clearly more people are annoyed. Not to mention, you may make some friends with the other patrons and this will give you more practice with "random people in public places."
When actually confronting, I don't think there's anything I can add that Jess didn't already say.