This paper reports on a study of adult migrants' L1 and L2 proficiency after extensive residence abroad, focusing on the predictive power of maturational and usage-based accounts respectively. The former perspective assumes age-related constraints on adults' capacity to become proficient in an L2, while the latter argues for the importance of environmental factors. The study adds a novel dimension to this debate by considering both L1 and L2 development. German speakers in Ireland completed German- and English-language tasks and responded to questionnaires. The data provide evidence of a moderate amount of L1 attrition, a high degree of L2 attainment in English, attrition in other L2s and a great amount of inter-individual variability, challenging both the monolithic view of L1 proficiency, and the deficit view of adult second-language acquisition. Although several variables were found to be influential, the findings overall support a usage-based account. Thus, in migration, adult bilinguals have the potential to develop both native-like proficiency in L2, and maintain their L1.