The Penal Laws
In homogenizing the mixed origins of the Anglo-Irish, the one decisive factor was Anglicanism; membership of the Church of Ireland was an absolute prerequisite for advancement or power. The key mechanism for the retention and reinforcement of this power was provided by a whole series of measures, known collectively as The Penal Laws.
In theory, these placed fanatically detailed restrictions on the property rights, social rights and religious practice of non-Anglicans, denying them, for example, the right to take leases or own land above a certain value, outlawing Catholic clergy, forbidding higher education and entry to the professions, and imposing oaths of conformity to the State church, the Church of Ireland.
Thorough enforcement of many of these laws was never a practical proposition, given the make-up of the population, but they nonetheless had a profound effect. To take one area alone, by the time the laws began to be relaxed somewhat in the 1770s, only 5% of the land of the country remained in Catholic hands.