Mix blackberries, sugar and water in a small saucepan and bring to a boil over medium high heat. Let bubble for 5-7 minutes, stirring every once in a while. Cool slightly and poor through a fine mesh sieve into a bowl, pushing down on the fruit with the back of a large spoon so as to squeeze out all the juice. Allow to cool and store in an airtight container in the refrigerator for up to 6 months.
For Irish Blackberry Lemonade Cocktail
Fill 10-oz cocktail glass with ice and add Jameson whiskey and simple syrup. Fill to the top with lemonade and a squeeze of fresh lemon juice. Garnish with lemon slice (and a little fresh mint would be good too!).