The explosion of Gaeilge (Irish)-medium primary and secondary schools has played a crucial role in the rebirth of our native language in Ireland. The popularity of attending Gaeilge-medium education is significant, and continues to increase annually (Gaeilscoileanna Teo 2008). However, the majority of Gaeilgeoirí (students who learn through the medium of Gaeilge) face an imminent transition to English-medium mathematics education, be it at second or third level education. This paper presents a theoretical model for investigating the transfer between different languages for learning mathematics and for interpreting the findings emerging from the Irish context. Key findings include that Gaeilgeoirí at secondary level experience a disadvantage when assessed through the medium of English; at both transitions Gaeilgeoirí experience difficulties with the syntax, semantics, and mathematics vocabulary of the English mathematics register; and at third level Gaeilgeoirí are unaware of the language difficulties that they are experiencing.