Background: Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is a disabling and complex chronic disease of unknown origin, whose symptoms, severity, and progression are extremely variable. Despite being relatively common, the condition is poorly understood and routine diagnostic tests and biomarkers are unavailable. There is no evidence on the economic impact of ME/CFS in Ireland. Methods: Adopting a patient and public involvement approach, we undertook three semi-structured focus groups, which together included 15 ME/CFS patients and 6 informal carers, to consider costs related to ME/CFS in Ireland, including how and why they arise. Focus groups were audio-recorded and transcribed verbatim, and we employed thematic analysis following the approach set out in Braun and Clarke (2006). Results: Themes from the data were: (1) Healthcare barriers and costs; (2) Socioeconomic costs; (3) Costs of disability; and, (4) Carer-related costs. Patient participants described a range of barriers to effective healthcare that led to extra costs, including delays getting a diagnosis, poor awareness/understanding of the condition by healthcare professionals, and a lack of effective treatments. These were linked to poor prognosis of the illness by participants who, as a result, faced a range of indirect costs, including poorer labour market and education outcomes, and lower economic well-being. Direct extra costs of disability were also described, often due to difficulties accessing appropriate services and supports. Informal carer participants described a range of impacts, including time costs, burnout, and impacts on work and study. Conclusions: The data suggests that ME/CFS patients face a wide range of costs, while there are also wider societal costs in the form of costs to the health service, lost productivity, and impacts on informal carers. These results will inform ongoing research that aims to quantify the economic burden of ME/CFS in Ireland and raise awareness of the illness amongst healthcare providers and policymakers.