Your foci seem to be upon Celtic origins of this name but for those with Slavic/Jewish /Eastern European origins the name/word Shanno is Polish meaning straw or hay. It is surmised that those with this last name were probably and originally involved with (either/both) the growing of/or merchandizing of straw or hay. The truncated American version is Shane. Not McShane or O'Shane ,but Shane. Some also continue with the original: Shanno. There is a Shanno trucking company whose origins are Eastern European.