The numerical ability of Irish students in international and national assessments is weaker than for literacy, particularly among higher-performing students. The aim of this study was to design a national teacher professional development programme to encourage teachers at post-primary level to develop numeracy competency across the post-primary curriculum. The project explored how numeracy and mathematics can be integrated meaningfully and constructively in a range of carrier subjects and explore how these subjects can, in turn, influence the manner in which relevant concepts can be addressed in mathematics lessons. The theory underpinning the effective design followed a model of professional development incorporating four strategies. The study was conducted over three school years and the pedagogical approaches adopted by teachers in the cross-curricular integration of numeracy are presented. How the approach supported teachers' learning is also presented according to a model of teacher agency in professional development and school reform, examining (1) individual practice; (2) outcomes; (3) perceived work context; and (4) school reform. The discussion points which arise describe the community of practice that evolved and the readiness of the participant teachers to embrace professional development in preparation for curricular reform.