Anti-Catholicism forms part of the dynamics to Northern Ireland's conflict and is critical to the self defining identity of certain Protestants. However, anti-Catholicism is as much a sociology process as a theological dispute. It was given a Scriptural underpinning in the history of Protestant Catholic relations in Ireland, and wider British Irish relations, in order to reinforce social divisions between the religious communities and to offer a deterministic belief system to justify them. This text examines the socio-economic and political processes that have led to theology being used in social closure and stratification between the 17th century and the present day.