Preheat the oven according to your sugar cookie dough recipe. Line baking sheets with parchment paper.
Lightly flour your work surface and roll the chilled sugar cookie dough to 1/4 inch thickness.
Cut 18 to 24 round cookies using a 3 inch round cutter, gathering and rerolling scraps as needed.
Arrange the cookie rounds on the prepared baking sheets, leaving a little space between each.
Bake the cookies according to your sugar cookie recipe instructions, until the edges are set and just starting to turn light golden.
Let the cookies cool on the baking sheets for 5 to 10 minutes, then transfer them to a wire rack to cool completely.
Divide the royal icing into three portions if not already separated: leave 1.5 cups white, tint 1 cup green with gel food coloring, and tint 0.5 cup to a flesh tone for the noses. Stir each until smooth and evenly colored.
Fit piping bags with small round tips and fill with the green, white, and flesh-colored icings, or snip small openings in disposable piping bags.
Use the food safe black marker to lightly sketch a guideline on each cooled cookie, dividing it into roughly three horizontal sections: top third for the hat, center for the nose, and bottom third for the beard.
Pipe an outline for the hat on the top third of a cookie with green icing, then flood the inside of the outline with more green icing, using a toothpick to gently nudge the icing to the edges if needed.
Pipe an outline for the beard section on the bottom third of the cookie with white icing, then flood the beard area with white icing, keeping the center area under the nose free if desired.
Pipe a small dome of flesh-colored icing in the center where the hat meets the beard to form the gnome’s nose.
When the hat and beard icing are slightly tacky, gently lift a tiny spot of the beard icing if necessary and press the nose area so it rests neatly between the hat and beard.
Allow the base layers of icing on the cookies to set until they are firm enough to hold detail, about 30 to 60 minutes at room temperature.
Once the white beard icing is set, pipe thin vertical lines of white icing over the beard area to create texture, leaving small gaps so the lines resemble strands of hair.
Pipe a clean border around the hat edge and brim with green icing to define the hat shape and cover any rough edges between colors.
If using shamrock icing transfers, place one shamrock onto the hat area of each cookie while the green border or a small dab of green icing is still soft so the transfer adheres.
Repeat the decorating process for all remaining cookies, working in batches so the icing does not crust before you finish each step.
Allow the decorated cookies to dry completely at room temperature for 2 to 4 hours, or until the royal icing is fully hardened to the touch.
Store the finished cookies in a single layer or between sheets of parchment paper in an airtight container at room temperature until ready to serve.