Welcome to the board!
A day trip to Dunn's River Falls near Ocho Rios... It is more like a very early morning departure and then you get back after dark trip. Dunn's River can get very crowded with all the cruise ships coming into Ocho Rios. I don't think you want to spend six plus hours in a car during your vacation!

I would recommend saving Dunn's River for your next trip to Ocho Rios. Instead head over to Mayfield Falls which is only a short distance from Negril and IMHO a much better investment in time on your vacation. Mayfields is FAR LESS congested with tourists. Do a search for trip reports about Mayfield Falls and plenty will come up. Mayfiled Falls is a 1/2 day adventure.

If you insist on Dunn's River....