For centuries Catholicism and Protestantism have been defining elements of identity in Ireland. This chapter surveys the Anglian Church of Ireland and the Irish Roman Catholic Church from the Act of Union in 1800 to the Belfast Agreement in 1998. For much of this period, there was a clear intertwining of religion and political allegiance. The Act of Union defined the relationship between the British state and Ireland until 1920 when the Government of Ireland Act partitioned the island and created a predominantly Protestant unionist six-county Northern Ireland. The following year the Anglo-Irish Treaty granted the largely Catholic nationalist Irish Free State dominion status on the same footing as Canada. This period of momentous political change witnessed the steady decline in the status, influence, and demographic strength of the Church of Ireland and the reconstruction, consolidation, and then ascendancy of the Catholic Church. Both churches feared the implications of new political arrangements for their religious freedom and key interests. The disestablishment of the Church of Ireland in 1869 saw it lose its privileged status and links with the British state. That experience was mirrored by the Catholic Church in the closing decades of the twentieth century when, for a variety of economic and social reasons, it lost the hegemonic status it had enjoyed since Irish independence.