This qualitative case study critically analyses Global Citizenship Education (GCE) teaching and learning practices within the situated context of formal education classrooms in Ireland. Specifically, the study focuses on distinctive characteristics of GCE practice, its participatory pedagogies and the role of dialogue and discussion within GCE. Previous studies expose certain weaknesses within formal education approaches to GCE that undermine its criticality and transformative intent (Biccum, 2015; Selby & Kawaga, 2014). However, classroom-based research is a significant gap within the field (Bryan & Bracken, 2011; Sant, Davies, Pashby, & Shultz, 2018) and teachers’ and students’ experiences remain underrepresented and under-researched (Bamber, 2020). Consequently, we have insufficient understanding of how GCE intersects with formal education at the point of practice. This is an important gap to address, in order to identify challenges and possibilities for more critical forms of GCE in the classroom. The study draws on participant observation across three post-primary classrooms, three semistructured teacher interviews and five student focus groups, in order to develop rich descriptions of GCE classroom practice. Underpinned by critical pedagogy and social constructivism, and framed against the concepts of voice, power and dialogue, the analysis offers new empirical evidence from the classroom into GCE teaching and learning and teachers’ and students’ negotiations. The study contends that the intersection of GCE practice and formal education classrooms can generate pioneering practice, yet also reveals tensions for teachers and students. In addition, I argue that participatory forms of GCE serve as an important counterpoint to prevailing classroom encounters that can result in transformative experiences for students. I draw on evidence to advance a typology of GCE practice that illustrates the importance of pedagogy, purpose and person to the participatory intent of GCE. Furthermore, the study establishes the centrality and distinctiveness of discussion for GCE teachers and students, yet also identifies problematic interpretations that limit the potential for more critical forms of dialogue. Implications arising from this study highlight the urgent need to develop ongoing, tailored and appropriate professional development for teachers that responds to their needs and existing expertise. Moreover, it is important to position young people to the fore of GCE practice, research and policy-making, in order to examine and understand GCE as a learning process that is conceptually grounded and concerned with cognitive and social development.