The Tudor conquest, colonization, and commodification of Ireland engendered large-scale Irish migration to Spain. Upon initial arrival, the Irish assimilated on explicitly racist grounds. In Michael T. Bailey's On the Pig's Back, the significance of Irish participation in the racial infrastructure of the Spanish Empire to the wider histories of slavery and white supremacy is advanced first with an analysis of seventeenth-century Castilian literature, Irish historiography, and Hiberno-Spanish case studies.

It was, however, during Spain's eighteenth-century Bourbon Reform era that Hiberno-Spaniards played an instrumental role in the empire's embrace of racial capitalism. In Madrid, powerful Irish reformers successfully pushed for the emulation of British policies such as agrarian capitalism, free trade within the empire, and political-economic statecraft. In the strategic borderlands of the greater Gulf Coast, from Florida to Yucatán and inclusive of Cuba, a wide network of Hiberno-Spanish governors, slavers, priests, and generals were also essential to the aggressive expansion of settler-colonies and plantation slavery-transforming the region into the world's center of slavery and capital accumulation by the early nineteenth century.

On the Pig's Back thus explains why the histories of racial capitalism, the Spanish Empire, colonial North America, and the Irish diaspora cannot be told without Hiberno-Spaniards.