The provision of immersion education and the sustained growth of Irish-medium education (IME) is a key developmental strategy of the Department of Education (DOE), which commits to “extend Irish-medium provision” as a national strategic action (Department of Education, 2021, p. 19). While anecdotal evidence suggests that attrition is an area of concern in immersion education contexts internationally (Mac Cárthaigh, 2019), there remains a lack of research studies on attrition and retention models which investigate this issue in post-primary IME in the Republic of Ireland (ROI). The researcher sought to address this dearth of information by collecting and analysing the perspectives of IME students on completion and non-completion in the Irish context. In particular, this research investigates why: (i) some students transfer out of IME to attend an English-medium alternative, and; (ii) some students consider transferring out IME, but ultimately choose to remain in IME, and; (iii) some students never consider leaving IME. This mixed methods study was conducted over two phases. A quantitative survey was completed by a sample of IME students (n=132). This was followed by focus groups with students (n=15) currently enrolled in IME and interviews with students (n=5) who had moved from IME to English-medium schools. Participants shared their experiences of and attitudes towards IME, their perceptions of education and discussed the factors which influenced their decision to leave or remain in IME. Thematic analysis (Braun & Clarke, 2006) was used to analyse the data generated (Mason, 2009). The findings were used to construct a new completion and non-completion model, building on existing international attrition & retention models, but which is specific to immersion education. Additionally, recommendations are provided for the future enhancement of IME provision and to help maximise completion rates in immersion education, both nationally and internationally.