Combine flour, cornflour, salt and icing sugar together in a bowl; rub in butter until mixture is combined and comes together in a ball. Wrap shortbread dough in clingfilm and store in fridge for 1 hour.
Press dough out on a very lightly floured work surface to about 5mm thick; stamp out biscuits with a shamrock-shaped cookie cutter and place on an ungreased baking tray.
Bake biscuits in preheated oven for 10 minutes, or until they start to brown. Lower oven temperature to 120 C / Gas ½; continue to cook for a further 30 minutes.
Transfer shortbread biscuits to a wire rack until completely cooled.