As years passed the two groups grew more and more into their respective areas that they saw comfort in through the form of majority. Catholics generally kept to themselves in the land we now see as the Republic and Protestants in the land we now see as the North. Though personally I would not use the term 'enemies', as although there were sectarian assassinations on both sides throughout our history the large part of the fight has been between the two major ideologies, Republicanism/Nationalism and Unionism/Loyalism. I will not delve deep into all that as it would be straying from the question but the most concise answer is simply that these groups just wanted to be around the people that has their same beliefs, which we can see up to today. It is even said that when the 'Troubles' began the Army Council of the Irish Republican Army held a decision of whether to aid just the Catholics or the entire 'Working class'. They eventually settled on providing that care to the Catholic minority though