The barrier around San Andreas is unlimited, means you can go straight east towards Liberty City, but never arrive there.Just go, go, go, get bored and kill yourself.Tho, there is a topic around the forum about a secret island, not south-east of LS, but south of Bayside Marina.Don't remember well where the topic was, but it should be around here.
You'll read more about this island in the topic.
You mean that island that's on that map in Lil Probe Inn and Toreno's place, but not on the actual game map?