Complex sentence: It should be noted that Howard regarded Conan's Cimmerians as ancestors of the Celts, which include the Gauls-which could make Eithriall a remote descendant, or a later reincarnation, of Conan.
Split into simpler sentences: It should be noted that Howard regarded Conan's Cimmerians as ancestors of the Celts, which include the Gauls. It would fit Howard's general scheme to consider Eithriall a remote descendant, or a later reincarnation, of Conan.