Will,
There are emergency medical services available in Negril, both public and private ambulances as well as some hotels having their own trained staff skilled on defibrillation units. Should you have a heart condition, please contact your hotel to see if they have one on premise to handle your situation. Not all have them, but several do.
Seastar Inn happens to be one of them, and there was a situation a few weeks back where the staff had to take care of a guest who passed out. They already had started to evaluate the need of the defib as the guest came to. Just a little too much "fun" in this case. Everything turned out fine.
Since you have not make it clear what "emergency" situations you are talking about, base it on what you would do in your own town. If a visit to clinic would suffice, then it will suffice in Negril as Accompong has stated. If you need advanced hospital emergency treatment, as you stated in your original post, there is not yet a hospital in Negril so you will need to be taken elsewhere.
The nearest hospitals are in Sav about a 25 minute drive away, not an hour and a half. Royal Medical probably has the best facilities and are privately run. But they do cost more than the government run public hospital in Sav. The public hospital also has emergency room services that have treated many a friend of mine when they were in a critical accident.
Cornwall Regional Hospital in Montego Bay is the one that is 1 and half hours away. They have the best facilities on the western end of the island.
Recently a person fell off the cliffs between Rockhouse and 3 Dives and ended up near the bottom by the sea. The ambulance was there in minutes and the doctor on call originally suggested taking them immediately to Sav for emergency treatment since it was the closest facility. But since they were conscious when the emergency workers got down to them, after speaking directly to the person by cellphone, the doctor asked them a series of questions and determined that they were in good enough condition to head to the Cornwall Hospital in MoBay.
After two days of MRI's and cat-scans and finding no serious damage, they were bandaged up and sent on a plane back home. They were extremely lucky as they had fallen some 25 feet. But the professionalism of the emergency crew and doctor on call made all the difference for them. I witnessed the entire series of events and the Negril rumor mill was in such fast action that by the time I walked back to 3 Dives (about a minute away), it had already been announced that the person had died. So much for the rumor mill...
I have heard stories from different visitors who both praise or complain about the emergency treatment they received in Jamaica. The one simple thing I would like to point out is that those that are complaining are still around to complain about it...