That will be cutting it awfully close - according to this website, sunset on the 24th is at 6:08PM, and you'll want to be there at least 15min before that. http://uk.weather.com/climate/sunRis...egril-JMXX0009
If you could move up your pickup time by 1/2-hour you'd be better. Remember, if it's taxi from Ivan's, they might have a few other dinner guests to pickup after they pick you up. You don't have to change the reservation time - you'll just be at the bar taking sunset pics and having a drink until then.
The worst thing would be to be stuck in the taxi as the sun goes down, and you miss your first sunset in Negril completely. Once you get to your hotel, you'll know better if you have the time to get to Ivan's in time. If it looks like you can't be early enough, call Ivan's and delay it a little - watch the sunset from your hotel beach - then go to dinner afterward. Sunset from the beach is a spectacular show, too!![]()