The Oxford Names Companion, OUP "Scottish Forenames" - Donald Whyte, FGH, FSG
Notes:
Scottish. Seònaid can be Anglicized as Seona or Shona and is the Gaelic form of Janet [q.v.] or the Irish Gaelic name Sinèad. The ultimate derivation is from Hebrew "God is gracious" via variants of John.
Thanks to Donald Whyte who provided much of the initial information for this site.