Line an 8-inch × 8-inch baking dish with parchment paper or tin foil and then set aside. Be sure to let the liner hang over the sides so you can easily lift the fudge out of the pan later.
Pour condensed milk in a microwave-safe bowl and heat for 1 minute.
Place white chocolate chips in a large glass bowl and pour warm condensed milk over the chips. Let sit for 1 to 2 minutes. Stir mixture until chocolate is fully melted and incorporated with the milk. If needed, microwave the chocolate for 15 seconds and then stir again.
Add Irish cream to white chocolate fudge and stir until completely incorporated. It will take a couple of minutes. Add in 3/4 of the chopped pistachios and stir to combine.
Pour fudge your prepared pan. Sprinkle reserved pistachios over the top and gently press into the top of the fudge.
Place fudge in your refrigerator for 3 to 4 hours until firm. Once the fudge is set remove it from the dish, peel off the foil (or parchment paper), and cut into 25 even pieces. Store in an air-tight container in the fridge.