When you say "Jamaicans" do you mean the current population of Jamaica? Jamaicans are people whose ancestors originate from 52 different ethnic groups. Now Jamaicans have built their own identity and culture that is highly distinct from everywhere else in the world, so Jamaicans today originate from Jamaica. I know it's not the most original reply, but a it's a rather precise one :)

Negril is relatively dry compared to other parts, as it's mostly flat where the beach is, so there is not as much rain there. With the seasons it's never easy to guess though, as there can be some tropical storm in the area and the result might be either that it will "suck in" all the clouds and rain elsewhere, but Jamaica will have sunshine all day long OR that some "arm" of that storm might be over Jamaica, which would mean a few hours of rain or just overcast. In general, first week of September is usually pretty good. It's mid October - November when it gets a little bit more wet.