This paper examines the relative importance of upper secondary subject choice and attainment in explaining social inequalities in access to higher education (HE) in Scotland and Ireland. These two countries differ in the extent of curriculum differentiation, in the degree of standardisation in school examination and in HE entry criteria. In particular, in Scotland subject choice in upper secondary education is more differentiated (both in terms of number and type of subjects taken) and allocation of places in HE is less standardised and more dependent upon the subjects studied at school than in Ireland. Given these institutional differences, we expected subject choices to be more important for explaining social origin differences in HE entry and access to prestigious institutions in Scotland than in Ireland. Because of increasing student competition for HE places, we further hypothesised the growing importance of school subjects over time in mediating social inequalities in HE entry in both countries, more so in Scotland than in Ireland. Our results confirm that subject choice is a stronger mediator of social inequalities in HE entry and access to prestigious universities in Scotland while attainment is more important in Ireland. Contrary to our expectations, the role of subject choice in HE entry has not become more important over time. However, in Scotland subject choice continues to be a strong mediator for social inequalities in HE entry.