There are indeed multiple ways one can bypass the barrier, including one way that's slightly less reliable, but requires no interaction with the main story's characters. It's something of a fail-safe in the event the player chooses to ignore them entirely, or some other circumstance prevents progress during a playthrough (as is the case here for Jack's quest being temporarily put on hold in favor of Misty's.)
If you'd like a hint on how to find the always-available entrance... hmm let's see... you may wish to pay special attention to the names of seemingly insignificant NPCs and see if you can find any that are different from the majority. Particularly on the South end of the island.
If you need better hints, or want me to tell you outright, I can oblige. I just prefer to let folks discover it a bit for themselves first and give out clues unless they insist otherwise.