Environmental Education (EE) remains marginalised in Irish primary schools despite its formal inclusion in the curriculum. This study investigates the barriers to EE integration and explores how Universal Design for Learning (UDL)-based Science Festival events can support teachers and students in overcoming these challenges. Survey and interview data reveal that EE is deprioritised due to curricular pressures, time constraints, conceptual misunderstandings and logistical concerns. A UDL-informed 'H2O Heroes' Science Festival event, was implemented to enhance EE accessibility and engagement. A 6-step UDL redesign process (reflect, plan, design, implement and evaluate, test and refine, reflect) is outlined which can be applied to other EE programs and STEM outreach initiatives. Findings indicate that hands-on, experiential learning significantly improved both teacher and student engagement. Teachers valued opportunities to observe innovative pedagogical approaches, while students demonstrated notable increases in environmental knowledge, particularly regarding water sources and pollution. The study underscores the potential of experiential, inclusive STEM outreach to bridge gaps in EE delivery. It highlights the need for continued partnerships between schools and outreach providers to embed EE more effectively in the curriculum, fostering environmental literacy and sustainability awareness among young learners.