Make the soda bread: Set an oven rack in the center of the oven. Preheat oven to 450F.
Sieve dry ingredients into a large bowl; add currants and stir to lightly coat. Make a well in the center. Pour most of the buttermilk into the well; use a claw-shaped hand (or a normal wooden spoon) to gradually mix the flour into the buttermilk to form a soft, slightly sticky dough. If needed, add more buttermilk, 2 tablespoons at a time, to bring the dough together. Tip dough onto a flat, well-floured surface. Wash and dry your hands (to prevent the dough from sticking to them). Pat your loaf into a round about 1 1/2 inches deep and gently flip it over. Score a 1/2–inch deep cross into the dough from end to end. Gently lift dough onto an oiled baking sheet.
Bake soda bread in the center rack at 450F for 15 minutes. Turn the oven down to 400F (don’t remove the bread) and bake 30 minutes more, until browned and fully baked. If you’re not sure, lift the bread from the baking sheet (using a clean kitchen towel or oven mitts) and tap on the bottom; if it sounds hollow, it’s done.
Make the full Irish breakfast: Line a plate with two layers of paper towels. Sprinkle tomato halves and mushroom cap with 1/8 teaspoon each salt and a few grinds of black pepper.
Heat 1 teaspoon butter in a medium skillet over medium heat; add pork sausage and bacon, making sure they have space between them. Cook bacon until crisped and rendered, 3 to 4 minutes per side; drain on paper-lined plate. Cook sausage, turning occasionally, until browned and cooked through, 6 to 9 minutes, depending on the thickness of the sausage. Add to paper-lined plate.
Add tomato halves, cut-side down, and mushroom cap to the skillet; cook until lightly browned and softened, 3 to 5 minutes. Transfer to a serving plate. (If you’re using black and/or white pudding or Boxty, warm them through over medium heat now, adding a bit of oil or butter to the pan to prevent sticking. Transfer to the serving plate).
If necessary, pour off all but 1 tablespoon fat from the skillet. If your skillet seems dry after frying the mushroom and tomato, add enough oil or butter to lightly coat the surface. Crack 1 or 2 eggs in the skillet; fry sunny-side, 3 to 4 minutes. (Cover the pan for the last minute of cooking if the whites closest to the yolk need help to set.) Add eggs to the serving plate.
Add bacon and sausage to the serving plate. Serve your Full Irish with buttered soda bread and lots of Barry’s Tea.
Have your oven ready to go when you make the soda bread; the buttermilk and baking soda will start to activate as soon as they intermingle, so this isn’t a dough that likes to sit around and wait. Do make sure your teaspoon of baking soda is a level teaspoon; too much baking soda in the bread will give your loaf a yellowish-green hue. Still delicious, though.
Optional add-ins for the full Irish include: 2 to 3 pieces black and/or white pudding; 1 to 2 sprigs fresh chervil or parsley, for garnish; or 2 pieces Boxty, or Irish potato pancakes (I like Darina Allen’s traditional recipe in The Guardian). I also like to use 1 cup leftover mashed potatoes mixed with 1 well-beaten egg, 2 tablespoons all-purpose flour, and 1/4 teaspoon salt. Form into golfball-sized rounds. Heat 1 tablespoon butter or olive oil in a medium skillet over medium-high heat until browned and cooked through, 2 to 3 minutes per side.)
This full Irish recipe is easily doubled. Use a large skillet and continue as directed with doubled amounts. Timing will generally remain the same, or about 30 seconds to 1 minute longer to cook.