Although Western mental health services are increasingly finding themselves concerned with assisting traumatized individuals migrating from other countries, trauma and posttraumatic stress disorder (PTSD) are under-detected and undiagnosed in psychiatric populations. This study examined and compared rates of traumatic experiences, frequency of traumatic events, trauma symptomatology levels, rates of torture, rates of PTSD and chart documentation of trauma and PTSD between (a) Irish and migrant service-users and (b) forced migrant and voluntary migrant service-users in Dublin, Ireland. Data were gathered from 178 psychiatric outpatients attending using a sociodemographic questionnaire, the Harvard Trauma Questionnaire-Revised Cambodian Version and the SCID-I/P. A substantial number of service-users had experienced at least one lifetime trauma (71.3%), and a high percentage of both the Irish (47.4%) and migrant groups (70.3%) of service-users had experienced two or more events. Overall, analyses comparing rates between Irish, forced migrant and voluntary migrant service-users found that forced migrants displayed more traumatic life events, posttraumatic symptoms, and higher levels of PTSD than their voluntary migrant and Irish counterparts, with over 50% experiencing torture prior to arrival in Ireland. The lifetime rate of PTSD in the overall sample was 15.7% but only 53.57% of cases were documented in patient charts. The results of this study are informative about the nature and extent of the problem of trauma and PTSD among migrant mental health service users as well as highlighting the under-detected levels of trauma among native-born service users.