Education is key to the structural integration of immigrants and their children. While research indicates that educational inequalities by migration background are a serious issue, relatively little is known about when, how and why they develop. The current paper adds to the literature by investigating to what extent achievement gaps by migration background develop during the primary school period. Drawing on data from a national longitudinal study of children in Ireland (N = 7577) and using structural equation modelling, it presents a path model of host language achievement. The results indicate that a large part of the disadvantages in English language achievement by migration background during primary school could be attributed to achievement gaps that already existed before school. Moreover, given earlier achievement, the effect of having a migration background was often insignificant or even positive, suggesting that disparities remain relatively stable or even decrease during the period of formal schooling.