[Bungleau]

Northwest of the windmill? That explains a lot...

Ishad N'ha is north of Valeia, and a little northwest of the Toad Village. Follow the river north until you come to a wooden pier. On the other side of the pier is a cave, and through there is the tunnel to Ishad N'ha and the Serpent Temple.

Note that you won't be able to get through the tunnel unless you've received the Mavin Ring. The first watcher is picky about that...

Seems to me that W&W came with a map (although I can't find mine right now) that gives you the overall lay of the land. Take a look at yours, and you'll get a better idea for where things are. You can also check Xignal's maps for a better picture as well.