Here are typical layouts for the iTunes folders:
If the "whole" library exists on the external drive you should be able to connect to it by holding down Option as you start iTunes, then opening the iTunes Library.itl file on the external.
If, in fact, you only have the media, then read this user tip. You can start the recovery process by creating a new empty iTunes library at the root of the drive, moving the media folder inside it, and importing the media to the library. The user tip should help make sure you don't lose any info when you sync the iPod touch.