The Oxford Names Companion, OUP "Scottish Forenames" - Donald Whyte, FGH, FSG
Notes:
Hebrew "ornament"; OT Genesis 4:19; 36:2.
According to Whyte, this name has never been popular in Scotland and is not a variation of Ada. However the ONC gives it as such a variant, without commenting on its level of popularity.
Thanks to Donald Whyte who provided much of the initial information for this site.