Whisk the eggs and sugar together in a metal bowl (I use my kitchen-aid bowl). And set it over a pan of simmering water.
Whisk the mixture until it reaches a temperature of 160 degrees (use an instant read or candy thermometer), it should take approximately 5 minutes. Whisk the entire time, being careful not to have scrambled eggs.
Remove bowl from the pan of simmering water and place on your mixer and beat with the whisk attachment at medium – high speed until it is cool to your touch(on the outside of the bowl) and very thick. This should take approximately 10 minutes.
While beating the egg and sugar mixture, melt your chocolate and cool to lukewarm.
In a separate bowl, combine the whipping cream and Baileys together and beat until stiff peaks form.
Pour the melted chocolate over the egg and sugar mixture and fold in until blended. Then fold in the whipped cream mixture. I will add a small amount of the cream mixture and blend it in (this will lighten up the mixture a bit) and then fold in the remaining until only streaks of white remain.
Place in a bowl, cover with saran wrap and chill till set, overnight.
Once set, it can be spread over a cake layer to be used as filling. Make sure you pipe a dam of your buttercream around the outer edges of the layers to prevent the mousse from being pushed out.
This is also excellent served as just a mousse dessert.