Preheat the oven to gas mark 3/170oC/325oF and line a 12-muffin/cupcake tin with baking cases.
To a mixing bowl or stand mixer bowl add the caster sugar and butter then cream the ingredients together until light and fluffy. This should take around 5 minutes. Add the oil and mix the oil into the butter and sugar.
Add the eggs one by one and combine. In a separate bowl, whisk together the sour cream and Irish cream and then pour into the main bowl that contains the butter, sugar and oil then continue to mix.
Sift in the flour and continue to mix then spoon the batter into the baking cases ensuring they are one third filled to allow room for them rise. Bake them for 20 minutes or until a cake tester comes out clean.
Once the cupcakes are removed from the oven, allow them to cool in the tin for 10 minutes then remove them and place them on a cooling rack to cool completely.
For the buttercream, use the paddle on a stand mixer or a hand held mixer and beat the butter until light and fluffy. This should take around 5 minutes. Gradually add the icing sugar until it is all incorporated then pour in the Irish cream and continue to mix until all the ingredients are combined and the buttercream is smooth. Prepare your piping bag with a star nozzle and pipe the buttercream onto the cupcakes.