In the twentieth century, the Irish-born population in England has typically been in worse health than both the native population and the Irish population in Ireland, a reversal of the commonly observed healthy migrant effect. Recent birth cohorts living in England and born in Ireland, however, are healthier than the English population. The substantial Irish migrant health penalty arises principally for cohorts born between 1920 and 1960. In this article, we attempt to understand the processes that generated these changing migrant health patterns for Irish migrants to England. Our results suggest a strong role for economic selection in driving the dynamics of health differences between Irish-born migrants and white English populations.