Line an 8"x8" or 9"x9" pan with a piece of parchment paper or foil.
Mix together all the ingredients, except for the walnuts, in a large saucepan and then turn the heat to low. Using a silicone spatula, stir frequently until no lumps of chocolate remain, being sure to scrape the bottom of the pan often.
It will take about 7-10 minutes. Once well combined, stir in the walnuts, if using.
Scoop the fudge into the lined pan and use the spatula to even it out.
Refrigerate for 3-4 hours or until firm before cutting into pieces.
Notes
Store in an airtight container for up to 1 week.
If you're gluten-free, make sure your Irish cream (and every ingredient you use) is gluten-free. I recommend the brand Carolans.