Everywhere !!!!! you picked the most perect place on earth for what you are looking for.
Bourban Beach, Alfreds and Roots Bamboo are the main beach bars. The Jungle is the club to catch the dancehall. Seastar on Saturday nights, Boat bar for afternoon reggae. You have to have an afternoon at Ricks . Check out Negril Escape too - they often have big named artists but not sure if they have anyone coming up. Ask the staff at the place you're staying - they will know of any additonal shows/parties .......be warned though, for a lover of Reggae and Dancehall Negril will be come very addictive