There is a Point of Interest on an island north of Sandycove Beach. Draw a straight line between this point and the western wall of the Forest map. Walk over to the wall, and you will be reach a bump-faced wall. If you look up, you’ll see a small waterfall.
Navigate up the rocks until you reach the waterfall. Now, walk through that wall! It is possible to walk south and come back out on the other side of the unjumpable chasm between the waterfall and the ledge to the south of it.
You can go further along, then right and back into the mountain face, or go back where you just came from and climp up the face that appears over the lava formation when you are facing north.
Keep jumping around and tell me what you find!