This is from a 3-ring binder cookbook from 1979: Duncan Hines Bake Shop in a Book. It has wonderful recipes that are still good and easy for today, though this one has a few extra steps.
1 package Duncan Hines Pudding Recipe White Cake Mix
1 package lemon pudding and pie filling mix
Few drops green food coloring
1 cup flaked coconut
Fluffy White Frosting (get a can of this, the recipe is too complicated!)
Make cake in 2 8-inch rounds. cook lemon filling as directed on package. Stir in green food coloring and coconut. chill well. Split each cake layer into 2 thin layers and spread filling between them. (Amy's note: Save yourself some trouble and just put it between the two big layers.)
Frost cake with fluffy white frosting. If desired, decorate top of cake with candy shamrocks, green gumdrops rolled out to form shamrocks, or with a circle of green tinted coconut.
My Grandmother celebrated St. Pat's more than she celebrated Christmas. She especially liked the parties at Bennigans with Brandy Alexanders. I don't think this is how they celebrate in Ireland, but it is a nice memory. Enjoy your St. Patrick's Day.