Cover your work surface with parchment paper and lay out the cooled cookies.

Separate the Royal Icing into two bowls.  Approximately 2/3 will remain white and the rest will be colored green using the gel food coloring.

Place 1/2 of the white royal icing in one piping bag and 1/2 of the green royal icing in the other bag.

Using the #2 round tip pipe a small outline around the outside of each cooking with the white icing.

Following the instructions for making icing to flood, thin the remaining white icing.  Spoon a small amount of thinned white icing into the center of the first cookie.  Used the spoon or a toothpick to spread the icing out.  Gently tap the cookie flat against your work surface to help the icing settle.  Repeat with all remaining cookies, one to a time.

Set cookies aside and let white icing dry completely before continuing.

Pipe a small outline of a shamrock onto the center of each cookie.  This does not have to be perfect.  It's easy to create with 3 heart shapes and a simple stem.  Add small dots of green icing around the outside edge of each cookie as well.

Thin the remaining green icing and spoon a small amount onto each cookie, flooding the shamrocks.

While the icing is still wet sprinkle with green sugar crystals.  The crystals will primarily stick to the wet green icing.

Set the cookies aside and allow the icing to harden completely before stacking or serving.