ok. i'm a woman, driven all over jamaica alone. the first time was a bit scary but after that it was a piece of cake.
there is no point getting a car just for there like sammy says.
best option is to get a car in mobay and return it to mobay.
private taxis are not cheap anywhere in jamaica, especially if you want to do nightlife. it's pretty wide open in portland.
once you drop the car at the airport in mobay you can venture out to negril.