I sure am glad I have read this thread. I am about to finalize a book about Alpine folklore, myths and customs and how they relate to the pre-christian Celtic times there, and to shamanism.
Anyway, I am writing this book in German, my mother tongue, and only knew of one German translation of Amergin's poem, where the "I am"-part ends with "I can shift my shape like a god" in the translation of Charles Squire. I am using that line as an indication that shape shifting played an important role in Druidry and was a common practise to experience other forms of existance, animated or not.
However, now that I have read a number of other translations, who tend to prefer the meaning that the God puts fire in the heads, I am wondering: how can it be that the translations are so far apart? Or, maybe I should ask, how did Charles Squire come up with this meaning?