There isn't a main concentration as such but several areas
a quick summary (apologies for missing or incorrect info)
SAN ANTONIO
theres the West End - a grid of streets with several bars and small clubs in them. Not got a good reputation but the drinks are normally cheaper and the clubs are largely free entry
Sunset strip - a small front up from the west end with 4 bars in a row (and 2 more just along the coast). Home of the newer Savannah & Sugar Sea, the legendary Cafe del Mar and the more popular Cafe Mambo
Just into the Bay side - houses the "superclubs" Eden & Es Paradis and there are a few bars along the seafront as well including Manumissions Bar M
SAN RAFAEL
On the road between San Antonio & Ibiza Town there are 2 more superclubs in Amnesia & Privilege (can also search for the intimate Underground club as well
IBIZA TOWN
The 3 superclubs are opposite the Dalt Vila old town - Pacha, El Divino & the newer Penelope. On the Dalt Vila side there are several inconnecting (& winding) streets with bars and some smaller clubs. At the harbour are the more well known bars in Base, Rock & K-ube
PLAYA D'EN BOSSA
Houses Space (probably the best club in the world) and Bora Bora beach bar.
Nearer the airport you will find the hugely popular DC10