There's two ways to get pass the barrier guarding the Werewolves Lair; either by siding with the Old Hermit or by siding with the Grand Oak. You can find the Old Hermit in the East section of the forest while the Grand Oak is located in the West. Despite who you aid, they will return the favor by giving you an item to allow you access to the place. If you want more information about the two and the quest you need to complete in order for them to aid you, you can find it in the Dragon Age wikia.
Or, if you meant how to get to the Lair inside the ruins, you basically need to do it the long way; the other entrance is deeper within the ruins by entering a underwater entrance if I recall correctly. Shouldn't be too difficult to find since it's a one-way sort of thing.