In GTA there are games within the game. Darts is one of the games that you can play with your friends or by yourself. Simply head over to the Irish pub designated with a bullseye symbol on the map and play a game. Aim for the middle 20 point narrow red strip located at the 12:00 position on the board. The narrow strip will triple your point value giving you 60 points. You will have to throw and land all three darts on this strip to score 180 points and then the achievement will be yours.