a motorized chain or cable guidance system would be easiest (assuming it's a straight shot).
How big is this barge? Is this fresh or salt
(tidal)? Does it load/unload from bow and stern (if it distinguishes these two) or does the same "end" of the barge always need to face land? Could you push/pull with a boat? Could you add a trnsom somehow and hang an outboard
Who owns the land where it "docks" on the mainland? Is there boat traffic in between the island and mainland?
Just trying to get a better picture of the equation.....