ABSTRACT In today's increasingly diverse societies, a key question is how to foster the structural integration of immigrants and their descendants. While research indicates that migrant educational underachievement is a serious issue, relatively little is known about achievement gaps at younger ages and in relatively new immigration countries. The current study sets out to estimate the size of disparities by migration background at age five (i.e. when they start school) and explores the causes of these gaps. It does so in a context that offers a compelling but under-researched case: the Republic of Ireland. It draws on the Growing Up in Ireland (GUI) data, a national longitudinal study of children in Ireland. The results suggest that some disparities by migration background already existed at the start of primary school, but also that gaps were limited to verbal skills and differed widely across groups. Moreover, social background only played a relatively minor role in explaining the differences, whereas the child's first language was a powerful predictor of disadvantages by migration background in verbal skills.