Make cookies. You will need two cookies per treat. Prepare dough and roll out dough 3/16 in. thick. Cut using second smallest (3 1/4 in.) cutter from set.
Invert whoopie pie pan and spray with vegetable pan spray. Drape dough over every other pan cavity, pressing to smooth surface. Bake 10-14 minutes, or until firm and golden brown. Cool pan on cooling grid 10 minutes. Carefully loosen bottom edges of pan using spatula.
Fill treats. Prepare crisped rice cereal treats. Press into sheet pan to 3/4 in. thick. Cut circles using smallest cutter from set. Sprinkle with shamrock sprinkles. Fill one cookie with one cereal treat.
Make piñata treat. Prepare royal icing. Use tip 8 and icing to pipe a line around rim of filled cookie; attach second cookie. Use tip 8 and icing to pipe an additional line around edge; roll in green sugar.
Decorate shamrock. Use shamrock cutter and black edible color marker to make a pattern on parchment paper. Use piping gel and a cut parchment bag to outline and fill in pattern; transfer onto cookie.