This paper uses a distinctive feature of the Irish education system to examine the impact of single-sex education on the gender difference in mathematical achievement at the top of the distribution. The Irish primary school system is interesting both for the fact that many children attend single-sex schools, and because these single-sex schools are part of the general educational system, rather than serving a particular socio-economic group. In keeping with research on other countries, we find a significant gender gap in favour of boys, but contrary to suggestions in the literature, our results provide no evidence that single-sex schooling reduces the gap. If anything, the gender differential is larger for children educated in single-sex schools than in coeducational schools.