Language massive open online courses (LMOOCs) represent an exciting prospect for language teachers and instructors around the globe (Bárcena & Martín-Monje, 2014). In this paper, we report on the dynamics of participation and learner behaviour in an ab initio Irish language course. The course, Irish 101, ran during March 2019, and we used a mixed-methods approach to analyse both typical patterns of behaviour among course participants and learner reflections upon their reasons for doing so. Findings suggest that most learners use the course resources in an assessing and exploratory manner and are far less likely to produce, or to examine, second language (L2) output, either written or spoken. Learners were found to be selective and to demonstrate significant metacognitive awareness (Wenden, 1998) in their interactions and learning methods, displaying agency and exploiting affordances beyond the design of the course itself. Implications for LMOOC design, including the need to question whether courses should emphasise L2 production or resource provision, are considered, in addition to a general need for more granular, dynamic research, so as to better understand the types of learners who engage in LMOOCs and to better cater to diverse learning needs.