Aspirin is an effective medicine for prevention of heart attacks in patients with coronary artery disease and works by preventing clots from forming. In previous studies aspirin has been found to be ineffective in between 2% and 65% of patients but none of these studies have looked specifically at coronary artery disease patients in Ireland. This study is being done to identify the percentage of patients in Ireland whose aspirin is not working effectively and help identify factors that could be used to target interventions to increase aspirin's effectiveness in Irish patients.