The big questions is if there is a big difference between the type of buttermilk/sour mild used? Anybody out there had experience with this stuff? One interesting note that the "The Joy of Cooking" makes is that milk from the store technically does not sour, it spoils, and that buttermilk from the store is not the same as buttermilk from heavy whipping cream and thus they require different recipes. In fact buttermilk is technically the byproduct of making butter, which if it (the butter) is made from heavy whipping cream is supposed to be much tastier than real butter from the store. To extend shelf life, the production process of the dairies sacrafice taste.
.... and so you might wonder why not just buy the soda bread from the store? .... because this is September and a friend is bringing a guest into her store on the 12th to talk about James Joyce.... and she needs Irish snacks.... and stores do not sell soda bread in September.... I suggested the usual (Guinness, Jamesons, Poteen, Bailey's) but that would require a sobriety checkpoint at the door....:)
Should you be interested my friend's store sells rare, used, and collectible books. The store has a couple first edition's of Joyce, Twain, and Upton Sinclaire....as well as some books which are over 300 years old.