Spanky, we did just that in February. Go to the Port Antonio board section here for Irie Always' (IA) trip report. I think she called "We Did It". We were in Negril for a week, and went to Mobay to meet Irie Always and her Hubby when they landed and then we headed up to Porty for 3 days. Wish we had had longer. It's absolutely beautiful. More jungly, more rain. Not the long beach you're used to in Negril, but more smaller beaches and bay type beaches. The people are ... well, Jamaican - warm, and friendly, but in PA they seem almost shy. Lovely, lovely people! The water is considerably rougher (people surf there! Not like Waikiki surfing, but surfing nonetheless).
You will need a car or a driver. It's not as populous as Negril and Mobay, and taxis are fewer and far between. And you need them! Attractions and activities are many, but far between! We went with a couple who have been friends with for years IA and happen to be tour operators. They don't usually go to that area and were on a fact finding "maybe we'll add this to our list" trip with us. The roads in Negril, BTW, are like fine German or American superhighways compared to PA! Yeesh!
We planned our trip well - made a list of what we wanted to see, mapped it out, went to furthest point and worked our way back. Worked well. If you want more info, PM me. We'd be happy to share our experiences!
It was well worth the trip. I'd do it again in a heartbeat!
My one item I would say to pack? Water shoes!