Irish soda bread is made with flour, buttermilk, baking soda, sugar, and salt. The traditional flour used for the recipe is a soft flour, the opposite of durum semolina. Soft flours are uncommon here in the US, but cake flour is a good mixture between soft and all-purpose that will work nicely if you are trying to closely approximate the historical recipe. I just use all-purpose, since that is what we buy in bulk and have for our longer term food storage/preps.
Buttermilk is the only hard-to-get ingredient. It can be found in most grocery stores and tolerates freezing well, but if you don't have a chance to run out and pick some up, it can be approximated by adding some vinegar or lemon juice to plain milk. Yet another (traditional?) technique for approximating buttermilk when none is available is to use the effluent from a somewhat rancid sourdough starter as the buttermilk portion of the recipe. I see no reason why the effluent, or whey, from homemade yogurt couldn't be used in this way. All four additives could change the base recipe to suit your tastes. Which is your favorite?