For the soda bread: Put the raisins in a bowl and cover with hot water. Set aside until softened, about 30 minutes. Drain and pat dry.
Preheat the oven to 375 degrees F.
Sift the all-purpose, cake and semolina flours, and the baking soda, cream of tartar and salt into a large bowl; stir in the sugar. Cut the 3 tablespoons butter into the flour until it resembles a coarse meal. Stir in the fennel seeds and the soaked raisins.
In a small bowl, stir the orange zest into the buttermilk, and then add to the flour mixture, stirring with a fork just until a dough comes together. Turn the dough onto a work surface and knead briefly until the loose flour is just moistened; the dough should look shaggy and uneven.
Form the dough into a 6-inch round and place in a 10-inch cast-iron skillet. Using a very sharp knife, score a deep cross into the top of the loaf. Bake until nicely browned and a skewer inserted into the center of the loaf comes out clean, 40 to 45 minutes. Remove from oven and brush with the 2 tablespoons melted butter. Cool for at least 30 minutes before slicing.
While the bread bakes, make the buttermilk-orange butter: Whisk together the buttermilk and orange zest in a small bowl. Put the butter and salt in a stand mixer fitted with the paddle attachment and beat until smooth. Slowly pour in the buttermilk mixture and continue beating until just incorporated. Spread into a ramekin, cover, and refrigerate for at least 1 hour and up to 24 hours to allow the flavors to meld.
Serve slices of warm or toasted semolina soda bread with the orange butter.