YLJ, just remember that Core Shamanism is a controversial topic at best in the Pagan world. Many feel he's been disrespectful in the way he stripped all cultures from the essential context in which these practises happen. I'm not saying I'm one of em, but be aware that there is a lot of criticism on his work becoz of this. Otoh, studying just the techniques certainly has merit as well.
Celtic Shamanism is one of those practices that have been derived from his teachings, as much of the lore that the celts practised has been lost, fragmented or fused with Christianity, making it very *very* hard to restore it to its original state. There's no real proof, to my knowledge that the celts even practised shamanism, though there are hints in the works of Taliesin and the like.
Some have indeed taken core shamanism and made a 'celtic variant'. Many reconstructionists just completely refuse to accept those traditions. It's also what gives it that 'wiccan' flavour, as Gardner basically did the same. He took Eastern elements, some stuff he'd learned from the British Trads and threw it together with the celtic pantheon.
All those traditions have their merit, even if they were founded only in the 1950's, but know what you practise and straigten out with yourself what it is that is important to you in your spiritual path.
Just my two cents