Start at the above screen and drop down the left side of the waterfall, pushing to the left the whole time. You may not be able to reach the first ledge, and it may take a few times, but it should work eventually.

Alright. I dusted off my game so I could give you exact instructions to this.

First, get to the waterfall. Then, once you've gotten there, set yourself up so you are on the right side of the waterfall, in the sliver of space that the waterfall and the line from the ledge makes. Test jump to see if you're still under the waterfall. Then, jump, and when you reach the top of your jump, jump again, landing on the first ledge. Continue jumping until you reach the above screen.

Once there, jump to the left side of the waterfall. Now, the water should stretch a good way from the end of the cliff. Walk to the edge of the cliff. You be standing right next to the waterfall. Jump in, and, since you have double jump, press left, down, and the b button all at once. You should be kicking to your lower left. You'll be moving very fast now, so be ready. Hold left the whole time you are falling. If you've done it right, Shanoa should rub against the ledge above Irina's and land on hers. Enter cave and save Irina.

You're welcome. Glad to be able to help.