Preheat oven to 375°F. Cream butter and sugar until light and fluffy. Beat in egg. Whisk together flour, baking soda and salt. Gradually add flour mixture into butter mixture. Mix until well blended.
Divide dough in half. Roll out half of dough at a time on a lightly floured surface to ⅛-inch thickness (approx. 12-inch circle). Cut into shamrock shapes with floured cookie cutter. Place ½-inches apart on parchment lined or unbuttered cookie sheets.
Bake 8 minutes or until lightly browned. Transfer cookies to wire racks; cool completely. Spread tops of cookies with buttercream icing and sprinkle with green cake decorating crystals. Store in airtight container.
Cream butter. Add powdered sugar and blend well. Stir in whiskey and beat well. Blend in food color. Add additional sugar or whiskey if necessary for spreading consistency. Makes ½-cup icing.
½ teaspoon of vanilla plus 2½ teaspoons water could be substituted for whisky.