This paper reports on performance data obtained as an integral part of a major research study on the effect of the transition from primary to post-primary education on Irish students' mathematics performance [V. Ryan, Making the Transition: A Student�s Mathematical Journey from Primary to Post-Primary School in Ireland, PhD. University of Limerick]. The data were obtained using a standardised test designed for use in Irish primary schools to assess student mathematical knowledge at the end of their first year of post-primary school. The results highlight significant underperformance across all curriculum strands of the official mathematics curriculum indicating an unsatisfactory grasp of mathematical knowledge at this stage of students' development. The item analysis also shows statistically significant differences in performance between male and female students. These findings are concerning in the context of major recent reforms of the national post-primary mathematics curriculum and targeted measures to avoid negative outcomes in mathematics as students make the transition from primary to post-primary education. In research terms, the end of year 1 in post-primary education is a neglected milestone in students' mathematics education that straddles a key period in students' mathematical journey when students negotiate academic and emotional transitions that affect their performance, attitudes and dispositions towards mathematics. This paper gives a recent appraisal of the mathematical knowledge of students entering year 2 of post-primary education in Ireland. It discusses factors affecting these student outcomes in mathematics in the context of the students' transition from primary to post-primary school/mathematics. Finally, it sheds light on a neglected mathematics milestone for students, and its potential to influence future student outcomes in mathematics.