Background: Government policy can promote physical activity (PA) as part of a multilevel systems-based approach. The Physical Activity Environment Policy Index (PA-EPI) is a monitoring framework which assesses the implementation of government policy by drawing on the experience of national stakeholders. This study is the first to assess the extent of policy implementation in the Republic of Ireland using the PA-EPI tool, and to provide information on how policy implementation can be improved, with the intention of maximizing its impact on population levels of PA. Methods: This mixed-methods research study, comprising eight steps, was carried out in 2022. Information documenting the evidence for implementation of PA policy, across all 45 PA-EPI indicators, was collected via systematic document analysis, and validated via survey and interview with government officials. Thirty-two nongovernment stakeholders rated this evidence on a five-point Likert scale. Aggregated scores were reviewed by stakeholders who collectively identified and prioritized critical implementation gaps. Results: Of the 45 PA-EPI indicators, one received an implementation rating of `none/very little¿, 25 received a rating of `low¿ and 19 received a `medium¿ rating. No indicator was rated as fully implemented. The indicators that received the highest level of implementation related to sustained mass media campaigns promoting PA and PA monitoring. Ten priority recommendations were developed. Conclusions: This study reveals substantial implementation gaps for PA policy in the Republic of Ireland. It provides recommendations for policy action to address these gaps. In time, studies utilizing the PA-EPI will enable cross-country comparison and benchmarking of PA policy implementation, incentivizing improved PA policy creation and implementation.