Gradually drizzle 1/2 cup Baileys. Mix until incorporated well.
In a separate bowl, mix your melted chocolate with about 1/2 cup of your whipped cream mixture to temper it. Stir until well combined. You will notice that, since your cream is cold and your chocolate is warm, your chocolate will be harder and harder to mix and will turn a little fudgy. That's ok.
Mix back your chocolate mixture, chunks at a time, into your whipped cream. You'll notice that as you start whipping, the chocolate will break into little pieces. And your mixture will be coarse and grainy - that's ok too. Just mix until everything is fully combined and until your cream starts to hold its shape.
Pour the Baileys mousse into your prepared pie crust and refrigerate overnight.