1. Irish Gaelic form of Jane (ONC). A correspondent [MA2] reports that Siobhán used as the equivalent of Joan. Another corresondent [MS] states that Johanna is the Anglicized form of Siobhan.
These usages are not reflected in our source books so any further examples would be welcome.
2. Irish and Scottish Gaelic form of Judith, from Hebrew "woman from Judea". (Whyte and ONC) Whyte also states that Siobhán can be Anglicized as Susan.