Siobhan was not used in France until 1980s, after 1981 (the statistics show 1988 as the first year with a minimum of three girls named Siobhan) and, even since then, the use has been purely marginal (less of seventy girls named Siobhan).
After the law 11 germinal, an XI (April 1, 1803), it was forbidden to pick a name outside an official list of names (that was, approximately, coincident with the saints' list); it was, then, impossible to use Siobhan in France (if some parents could have known it in a trip to Ireland, for example). The law was modified in 1966 to include several mythological names, regional names, compound names and some variants and diminutifs, but not foreign names; again, Siobhan was impossible.
I cited 1981 because then the law was again modified in order to accept, among others, foreign names.
As you pointed, Siobhan is not credible at all as name for a French girl in the 19th c.