Background: Adverse childhood experiences (ACEs) have various deleterious effects on mental health but few studies have been conducted in Ireland. Objective: The primary objective was to determine if there were significant differences in occurrences of ACEs in U.S. and Irish adults. We also sought to determine if there were unique associations between individual and multiple ACE events and mental health. Participants and setting: Preexisting nationally representative adult samples from the U.S. (n = 1893) and Ireland (n = 1020) were utilized for analysis. Method: To determine if there were significant differences in the occurrence of specific ACE events and the mean number of ACEs experienced by U.S. and Irish adults, chi-square difference tests and an independent samples t-test were used, respectively. Binary logistic regression was used to examine the unique associations between ACE events and major depressive disorder (MDD), generalized anxiety disorder (GAD), posttraumatic stress disorder (PTSD), and Complex PTSD (CPTSD). Nationality, sex, age, and educational level were included as covariates and adjusted odds ratios are reported. Results: Irish respondents had a higher rate of ACEs, were more likely to experience specific ACEs, and to meet diagnostic requirements for MDD, GAD, and CPTSD than U.S. respondents. Emotional neglect was more strongly related to mental health than all other ACEs, and there was an exceptionally strong dose-response association between ACEs and CPTSD. Conclusions: ACEs seem to be more common in Ireland than the U.S., and efforts to minimize exposure to ACEs through public policies may lead to beneficial mental health effects.