In a small saucepan, heat rum. Add raisins and simmer for 3 to 5 minutes or until raisins have absorbed all liquid. Transfer to a bowl. Let cool completely.
In a bowl, place cookie dough. Add rum raisins and mix thoroughly. Shape into a disc with your hands and refrigerate for 30 minutes.
With the rack in the middle position, preheat the oven to 180 °C (350 °F). Line a baking sheet with parchment paper or a silicone mat.
On a lightly floured work surface or between two sheets of parchment paper, roll out dough into a 10-mm (½-inch) thick sheet. Using a 6-cm (2 ¼-inch) fluted cookie cutter (or other), cut cookies, reusing scraps. Place on the baking sheet, spacing them evenly.
Bake one sheet at a time for about 15 minutes or until lightly browned. Let cool completely. Top with lemon royal icing, if desired.