The roots of the Troubles stretch back to the late seventeenth and eighteenth centuries,
when successive English and British conquests and reconquests of Ireland stripped the native Irish and old English Catholics of ownership of the land and replaced them with English and Scottish settlers.
Today, to be Catholic in Ireland usually signals descent from the native Gaelic Irish or the old English Catholics who persisted after the sixteenth century revolution.
To be Protestant signals likely descent from the colonial English and Scottish settlers.