Preheat the oven to 350°F. Line a 9x13 baking dish with parchment paper, set aside.
Place the brownie mix, Irish cream, oil, and eggs into a large bowl, mix per the directions on the back of the box.
Add the batter to the prepared baking dish and smooth out the top. Bake for 22-25 minutes until a toothpick inserted 2 inches from the side comes out clean. Or bake per the directions on the back of the box. Let cool completely.
While the brownies are cooling, make the ganache. Place the chocolate chips in a medium-sized heat proof bowl, set aside.
Place the Baileys and heavy cream into a small saucepot, stir occasionally until it just comes to a simmer. Pour this over the chocolate chips and cover the bowl. Let sit for 5 minutes.
Whisk until smooth. Let the ganache sit at room temperature until it is firm. You can also place it in the fridge for about 1 hour.
Place the ganache into a large bowl with an electric hand mixer or the body of a stand mixer with the whisk attachment and whip until it becomes light and fluffy.
Lift the brownies out of the baking dish by the parchment paper and spread the ganache evenly over the brownies.
Cut into servings, then drizzle with optional melted dark chocolate. Serve.
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