History of self-harm is the strongest predictor of suicide, but there are few national studies that estimate the risk of suicide following self-harm in a clearly defined clinical cohort. Records from the National Self-Harm Registry Ireland between 1 January 2015 and 31 December 2017 (n = 23,764) were linked to national suicide records via the Irish Probable Suicide Deaths Study. The 12-month cumulative incidence of suicide for male, female and all persons was 1.3%, 0.6%, and 0.9%, respectively. Suicide risk was more than 80 times higher in the self-harm cohort relative to the general population. Associated factors included male sex, older age, attempted hanging as a method of self-harm, and self-harm history in the previous 12 months. This national study highlights the greatly elevated risk of suicide mortality following hospital-presenting self-harm. These findings reinforce the need to provide appropriate care and timely interventions for this patient group.