The only place you will find a dress code is at a few of the restaurants at some of the AI's. If you are not staying at an AI, then no worries. At any other dining establishment (including Ivan's, Rockhouse, Charela or Kuyaba) a t-shirt and shorts will be fine.

I, personally, will throw on a collared shirt if we're heading to one of the nicer places mentioned above. If we're going to, say, 3 Dives or Just Natural then a t-shirt it is. One this is for sure - you'll never catch me wearing slacks or dress shoes in Negril.