It is with great pleasure that we present the following proceedings from the Ninth Conference on Research in Mathematics Education in Ireland (MEI 9), which took place in Dublin City University (DCU) in October, 2023. The conference theme for MEI 9 was Conceptualising Success in Mathematics Education. The discourse on success in mathematics education is populated with reference to student achievement and proficiency on standardised examinations – a distinguishing, tangible measurement taken to represent the success, or by association, failure, of a child, student, or adult, in their studies of mathematics. Scholars, however, contend that taken-for-granted ways of operating that privilege some individuals and exclude others should be uncovered and challenged to not just better understand, but to transform mathematics education. Therefore, MEI 9 served to provide a space to investigate what – and who – constitutes success in mathematics education, and how this is achieved.