Nothing to do with Irish, Welsh or Spanish.  It isn't of Irish origin, of course, though it found its way there, and to many other places, in some of which it became Black Jack Davy.  Lots more information (and speculation) in the many previous discussions archived here, which can be found in the way Ed suggested.  The "Digitrad and Forum Search" box is the place to start looking.  Have fun -it really is worth the effort.

Malcolm