Prepare Your Cookies: Ensure you have 24 Shamrock-Shaped cookies ready. If making your own, ensure they are fully cooled before decorating. (See My Easy Sugar Cookie Recipe for Fun Shapes).
Prepare Your Icing: Divide the white royal icing into 2 separate small bowls. Add 2 drops of green food coloring to one bowl of royal icing and stir to combine for the green icing. Leave the second bowl white. Optionally, for a third color, a lighter green, add a drop of green food coloring to another bowl of royal icing and stir to combine for the light green icing.
Fill Piping Bags: Fill the piping bags with the colored royal icing, creating one green, one white, and optionally one light green piping bag.
Decorate Your Cookies: Begin decorating cookies by outlining the shamrock first, then filling the centers and decorating with the desired design using the piping bags.
Allow to Dry: Allow the cookies to "dry" by ensuring they're placed in a single layer before storing.