The Oxford Names Companion, OUP Clan MacLean website and others
Notes:
Scottish Gaelic form of Beatrice/Beatrix, from Latin "bringer of joy".
ONC says Beathag is "often Anglicized as Sophia or Rebecca". Because of this, these two names and Beatrix/Beatrice are classed here as lesser synonyms.
Thanks to Donald Whyte who provided much of the initial information for this site.