• Preheat the oven to 350°F, then butter and line a 9-inch baking pan. Set them aside. 

For Streusel Topping:

• In the mixing dish, combine the oats, salt, and sugar.
• Then rub in the cold butter until it's completely integrated and the texture resembles coarse breadcrumbs.
• Set aside in the refrigerator while you finish making the cake.

For Irish Apple Cake:

• In a mixing bowl, cream together the butter and sugar until smooth and creamy.
• After that, beat in the vanilla extract, followed by the eggs one at a time.
• Combine the baking powder, flour, salt, and cinnamon in a separate mixing bowl.
• Along with the milk, fold the dry ingredients into the wet components.
• Once the butter has formed, pour it into the prepared pan and top with the sliced apples, ensuring that they are evenly distributed.
• Spread the streusel topping evenly over the apples.
• Preheat the oven to 350°F and bake for 75 minutes, or until golden brown and crunchy on top.
• Remove from the heat and set aside to cool slightly before removing the cake from the tin.
• Dust the cake with powdered sugar when ready to serve, slice, and enjoy!
• Keep the cake at room temperature for up to 4 days in an airtight container.