The Irish-born population in England is 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 health penalty arises principally for cohorts born between 1920 and 1960. This paper attempts to understand the processes that generated this migrant health pattern. The results suggest a strong role for early childhood conditions and economic selection in driving the dynamics of health differences between the Irish-born migrants and White English populations.