As a practicing Celtic shaman, I'm not sure how useful these cards are. The method for using them is confusing and not really enjoyable. The artwork is (in my opinion) pretty bland & amateurish. There really aren't a lot of people out there writing about Celtic shamanism; some people don't even believe that the Celts practiced shamanism at all! Seems like if I want a book on the subject, I'm pretty limited to John Matthews, & his work is not always very enlightening or organized. Such is the case with this deck.
You don't need this deck to practice Celtic or any other form of shamanism, but some better decks might be "The Druid Animal Oracle", "the Celtic Tree Oracle" & "the Greenwood Tarot." It doesn't have to say "Celtic Shamanism" on the box to be of use to you.
This deck is not a standard tarot meant for divination, and thus can be difficult to use well for that purpose. For a practicing Celtic shaman, they are indispensible for planning and focusing a journey into the Otherworlds.
If you are a Celtic shaman, this deck can become a truly sacred object. Those seeking to use it as a regular tarot should have some tarot experience first; even then, it may not work as you expect it to.