In a large mixing bowl, mix together the white cake mix, eggs, oil, 1 cup Cherry 7UP®, and 1/4 cup cherry juice. Mix just until smooth, then spoon or scoop into cupcake liners. I fill my liners 1/2 -2/3 full.
Bake at 325°F for 16-19 minutes. Allow to cool before frosting.
While the cupcakes are baking/cooling, make the buttercream! Place the softened butter, lime juice and Cherry 7UP® in a medium mixing bowl and cream together until smooth. Add in 4 cups of powdered sugar and mix. Continue adding powdered sugar until a stiff consistency is reached (5 cups was perfect for me, but depending on the how soft your butter is, you may need more or less).
Add 2-3 drops of bright green gel coloring to get the lime color - mix well.
Pipe the buttercream onto the cooled cupcakes; garnish with a cherry and sprinkles if desired.