Preheat oven to 325 F. Line 2 baking sheets with parchment paper.
In a large bowl, combine flour, sugar, baking soda and salt; mix well. Add eggs and 1/4 cup liqueur; mix well.
Cut dough into 4 equal pieces. Place 2 pieces of dough on each baking sheet, leaving space between them. Form a slightly rounded 2-1/2- x 8-inch loaf that is about 1/2 inch thick.
Bake for 25 minutes. Remove from oven and allow loaves to cool 5 minutes. Cut each loaf into 1/2-inch-thick slices. Lay slices cut side down on baking sheets and bake an additional 15 minutes. Turn biscotti over and bake an additional 15 minutes. Let cool.
In a small bowl, combine powdered sugar, Irish cream liqueur, mix well. Take a small amount of the prepared icing, place in a cup & add food coloring, mix well.
When biscotti is cool, place white icing in a piping bag fitted with a round tip. Pipe a line about 1/8 in from edge of biscotti. Flood inside with white icing. With green icing make 3 shamrock designs on each biscotti on top of white icing.
Melt chocolate & place in a small piping bag with a round tip. Outline the edge of each biscotti with a line of chocolate. Set aside to dry before storing in an airtight container.