I would have thought you'd been here long enough now to know that etymology doesn't always work that way.
Shana is ultimately derived from
John, so it has the same meaning. It is an Irish Gaelic and/or Welsh version of it, to be sure, but it's still ultimately the same name. It's not our fault, so don't attack us for it. If you don't like the truth that's not our problem.
If you've seen it listed somewhere as having a different etymology, we'd be interested to know about it, naturally, but you need to provide that information for us. It is possible that it did originally have another meaning in a Celtic language, due to cultural transfer at the contact point of Celtic and
Christian culture. In such situations the conquered Celts were often given Hebrew/Greek/Latin-based names by their
Christian conquerers, and often names were chosen which had either a similar sound or a similar meaning. Hence, girls named
Eileen were called
Helen, or girls named
Gladys were called
Claudia etc. However, if
Sian /
Sine /
Sean / Seon etc were originally a Celtic name set which was "adopted" as a version of the
Jane /
John name set, the original meaning is probably lost to us at this point.