Preheat the oven to 350 degrees F (175 degrees C). Butter the bottom of a 10 1/2 x 15 1/2-inch jelly roll pan.

Make the brownies: Empty both brownie mix packages into a large bowl. Add Irish cream liqueur, oil, and eggs; beat with an electric mixer until smooth. Spread batter into the prepared pan.

Bake in the preheated oven until brownies are set and a toothpick inserted into the center comes out clean, about 20 minutes.

Remove from the oven and allow to cool completely, 20 to 30 minutes.

While the brownies are cooling, make the frosting: Beat butter in a large bowl until smooth. Add Irish cream liqueur and beat until creamy. Slowly beat in confectioners' sugar, 1 cup at a time, until frosting is desired stiffness.

Spread frosting on cooled brownies. Cut into 30 squares.