Dunns River is a good excursion from CSS or CTI but CN is far away from it. You may want to look into a tour of YS Falls, Appleton Estate, and or the Black River instead and do Dunns River on another trip to one of the other 2 mentioned. That being said, Dunns River is a stream running down the hillside to the Caribbean with a series of waterfalls that you actually climb up on foot while in the water. Groups hold hands for balance single file, wear water shoes (I suggest you buy and take a pair rather than rent them...), and put their feet into natural and man made steps on the stream bed. If you bring a camera the guide can carry it and take pictures. Tips will be expected of course. Let me remind you that the water is cold as it is running down the mountain. At certain points you can take a plung into that cold water or stand under a small waterfall.
We were just at CN and would highly suggest going to Mayfield Falls over Dunns River. It is much closer and not as commercialized as Dunns River. There were 4 of us and when we got there had our own tour guide. There was a small group in front of us and no one behind. We have done Dunns River in the past and would pick Mayfield Falls over Dunns River any time!
My guess is it is about a 2.5 - 3 hour bus ride to Dunn's River Falls from Negril. Climbing the falls takes maybe 1 hour so you are spending the majority of your day on a bus. It would not be an included activity through Couples when you stay in Negril but it is when you stay at CTI or CSS in Ocho Rios. I would suggest skipping it this vacation and on staying at either CTI or CSS in Ocho on your next vacation when it is about a 15 minute bus trip and do it then.
Just back from CSA. Personally, agree with rckbrt above that going to Dunns River is not best from Negril due to the distance (3-hour drive each way). Save that for a CTI or CSS trip. Instead, I agree with the YS Falls and Appleton Rum/South Coast tour. We did that on 3 March, and it was a great day out!! YS Falls is absolutely beautiful, sensitively developed to preserve the gorgeous scenery, not overcrowded. Booked a private driver (yourjamaicantourguide dot com) before leaving home, the Your Jamaican Tour Guide company is properly licensed/insured, and Alrick Allen and Kissan made sure we had a great day on our own schedule. Their web site lists optional stops and the cost of entry to attractions. Note that the Pelican Bar boat chap wanted 25 USD per person (not per boat load) to go out to the offshore bar, so we skipped it but others may enjoy. Found this company via Trip Advisor reviews. Learned a lot about Jamaica, ate local fish at Border, always felt safe and well cared-for. BTW, the rum punch at the Appleton factory is awesome - and you can ask for seconds. Or thirds. Or.... (we weren't driving, so all good). Of course, you can also book tours via the CSA Tour Desk in the Great House lobby - I just wanted to book several weeks ahead as it was a birthday present.