Background Insufficient physical activity (PA) is a leading risk factor for premature death worldwide. Ireland's public healthcare system, the Health Service Executive (HSE), has supported the development of the National Exercise Referral Framework (NERF) to tackle low levels of PA amongst those with non-communicable diseases (NCDs). 'NERF centres' are medically supervised PA programmes across Ireland that have established referral pathways with local hospitals and general practitioners. ULMedX is one such NERF centre offering exercise-based cardiac rehabilitation (EBCR) with the aim of intervention development to reduce early drop-out and maximise adherence for optimal health benefits. Aim The purpose of this research was to identify the major factors influencing participants' adherence and early drop-out at ULMedX. Exploring areas for future development were also prioritised. Design & setting Qualitative interviews were conducted with long-term attenders and people who have dropped out (PWDO) from ULMedX. Methods Guided by the Theory of Planned Behaviour the 1-1 semi-structured interviews were performed, transcribed, and evaluated through thematic analysis. Results Analysis was performed on 14 participants (50% female; mean age 67.3 years), comprising long-term attenders (n = 7; 13-month duration, 64% of classes) and PWDO (n = 7; 2.8-month duration, 22% of classes). Three major factors affecting adherence and drop-out were identified: social support, perceived outcomes from participation and practical barriers to attendance. Areas for future development included the provision of evening and advanced classes, psychological support, more exercise variety, more educational seminars and new members start as their own group. Conclusion The findings suggest participants at ULMedX are more likely to have had a better experience and commit to the programme if they believed involvement would benefit their physical and mental health, increase their exercise motivation by engendering a positive attitude to exercise, and that the ability to attend was within their control. Future interventions at ULMedX should have their structures centred around these motives for engagement. ULMedX should also test the participant recommendations to overcome the common barriers to adherence.