Over the past decades public health research has presented compelling evidence that health is socially determined. To address structural inequalities and inequities in health, public policies require intersectoral development and implementation. Health Impact Assessment (HIA) is an established tool for analysing potentially detrimental health outcomes of policies, programmes and projects. In Ireland, we are presented with a unique opportunity to examine HIA implementation by capitalising on the synergy of recent policy commitments driving HIA implementation. Specifically, the national public health framework Healthy Ireland 2013-2025, and the newly developed all-island Institute of Public Health HIA guidance (2021), have coalesced in steering the strategic direction of HIA. Building on these policy commitments, Development of a HIA Implementation Model: Enhancing Intersectoral Approaches in Tackling Health Inequalities (HIA-IM) is an inter-disciplinary research project. It aims to further strengthen intersectoral policy and practice in tackling health inequalities through the development of an enhanced HIA implementation model. The implementation model will be developed by applying an action research methodological approach to the conduct of two HIAs in Ireland – one at a local policy level, addressing the Cork City Development Plan, and one at a national policy level addressing the Irish Government’s Climate Action Plan. HIA-IM will involve collaboration with key stakeholders to inform the policy direction of HIA implementation. Factors affecting HIA implementation will be investigated using a hybrid implementation framework combining the Consolidated Framework for Implementation Research (CFIR) and Normalisation Process Theory (NPT). A Community Engagement Toolkit for HIA will be developed as a practical resource to be used by those conducting HIAs. Research findings will directly inform the national roll-out of the Institute of Public Health Ireland HIA programme. The outcomes from this project will contribute to optimising the use of HIA in the development of intersectoral policy for population health in Ireland.