Sectarianism existed in ireland by design. Irish men on both sides of the religious divide wanted greater autonomy from London and this was not to the British government's liking and so they backed the most hardline group and fostered disunity through fear. When it came to partitioning ireland they left out counties from northern ireland that would have meant a catholic majority and so ensured there would not easily be a unification. These were common tactics of the British empire, which was probably one of the greatest empire building groups around. I think it would be naive to think these tactics are uncommon and isolated and not still in use they are after all very effective, the people are not attacking you if they are attacking each other