Background: Previous research has identified variation in the implementation of guidelines for managing presentations of self-harm or suicidal ideation to the Emergency Department (ED). A National Clinical Programme for Self-harm and Suicide-related Ideation (NCPSHI) was introduced in Ireland in 2015 which identified key components of care for implementation within the ED. Implementation outcomes were found to differ across hospital groups depending on pre-existing liaison services. Methods: A qualitative study was used to explore factors contributing to variation in the implementation of key components of care of the NCPSHI in Irish EDs. Thirty participants (n=15 hospitals) were interviewed about their experience of implementing components of care and the determinants of implementation. The Consolidated Framework for Implementation Research (CFIR) 2.0 was used to guide interview questions and code implementation determinants. Determinants were mapped to components of care. Findings: Biopsychosocial assessment was a consistently implemented component of care with few barriers reported. A timely and compassionate response was facilitated by collaboration between programme and ED staff and availability of a designated assessment room, both of which tended to be greater in hospitals with pre-existing consultant-led services. The involvement of family/carers was influenced by recipient preferences and the level of risk of self-harm behaviour where a person did not consent for family involvement. The provision of emergency care plans was affected by time constraints while provision of follow-up was influenced by handover between the liaison team and the availability of timely aftercare from community providers. Conclusions: This study builds upon our previous quantitative study of implementation outcomes of the NCPSHI, reinforcing biopsychosocial assessment as a consistently implemented component of care, showing improvements only for hospitals without designated services for self-harm before programme introduction. Established pre-existing liaison services enhanced the implementation of components that depended on local collaboration and resources with the inner setting of the hospital. Limitations in the timely availability of next care providers in the community warrants attention to ensure those who present to the ED receive appropriate follow-up and aftercare.