Preheat oven to 325°F Prepare 12 cup Bundt pan by spraying with nonstick baking spray.
Combine flour (3 cups), baking powder (1 teaspoon), and salt (1/2 teaspoon) in a small bowl and set aside. In a small liquid measuring cup mix Irish Cream (1/2 cup) and milk (1/2 cup) and set aside.
In a large bowl, using a hand-held electric mixer beat butter (1 1/2 cups) and sugars (2 cups brown sugar + 1 cup) at medium speed until fluffy about 6 minutes. Add the eggs (5), one at a time, and beat well to incorporate after each addition.
To the butter mixture, gradually add half the flour mixture then half the milk mixture. Repeat with remaining flour then milk and mix until incorporated.
Pour batter into prepared pan then smooth batter evenly. Bake in preheated oven 1 hour and 15 to 20 minutes or until a wooden skewer inserted into the center comes with a few moist crumbs.
Allow cake to cool in pan 15 minutes. Remove cake from pan to a plate or serving dish and cool completely.
Meanwhile, make the glaze by combining sugar (2 cups), Irish Cream (3 tablespoons), and 3 tablespoons of water, adding more water as needed, 1 tablespoon at a time, until smooth. Drizzle cooled cake with glaze