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 entailed two keynote presentations, two symposia, a panel discussion and a number of oral paper and poster presentations. 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. As part of MEI 9, the keynote presentations, which were delivered by Professor Susanne Prediger and Dr Niamh O’ Meara, and the panel discussion, involving Dr Thérèse Dooley, Dr Zita Lysaght, Dr Vasiliki Pitsia and Dr Gerry Shiel directly addressed the conference theme. These invited contributions ensured that MEI 9 afforded the opportunity for those working in the field of mathematics education to challenge existing conceptions and perceptions of success, and initiate new meanings, collaboratively developed, fitting with the needs of the informed, equitable, and socially-just citizen of the 21st century.