Old. Written Shan, Shane — the same Irish word, sean, in the spellings English gave it.
Old: it is the Irish sean. It is written Shan, Shane in English spellings of Irish names.
sean. English spellings of the same word include Shan, Shane, because the names were written down by ear.
Carrick-on-Shannon (Cora Droma Rúisc), Shannon (Sionainn), Drumshanbo (Droim Seanbhó).