Purpose: This study represents the first assessment of the prevalence of trauma exposure, and Posttraumatic Stress Disorder (PTSD) and Complex PTSD (CPTSD), ever conducted in the general population of the Republic of Ireland. Additionally, prevalence of past-year mental health service use, comorbidity with major depression and generalized anxiety, and risk factors associated with PTSD and CPTSD were assessed. Methods: A nationally representative sample of non-institutionalized Irish adults (N = 1020) completed self-report measures of trauma history, trauma-related psychopathology, mental health service use, and concurrent mental health problems. Results: Lifetime exposure to one or more traumatic events was 82.3%, and 67.8% reported experiencing two or more traumatic events. Males and females significantly differed in their frequency of exposure to eight of 16 traumatic events. The past-month prevalence for PTSD was 5.0% (95% CI 3.7%, 6.3%) and 7.7% (95% CI 6.1%, 9.4%) for CPTSD. Of those who screened positive for PTSD or CPTSD, 48.6% accessed mental health care in the past year. Comorbidity with major depression and generalized anxiety was high, especially among those with CPTSD. Several unique and shared risk factors for PTSD and CPTSD were identified. Conclusion: Approximately one-in-eight Irish adults met diagnostic requirements for PTSD or CPTSD, and comorbidity with other disorders was high. History of interpersonal trauma and exposure to multiple types of trauma in different developmental periods were associated with CPTSD. Many individuals did not access mental health care revealing a substantial mental health treatment gap.