To give a glib overview of history, in 1690--back in the days when the Holy Roman Empire really was an empire--Protestant William of Orange defeated Catholic King James II. For the next 300 years or so, Catholics of Ireland were discriminated against in one way or another because, as everywhere else, to the victors--those loyal to the British/Protestant throne--went the spoils. The small number of Irish who always demanded independence eventually became known as "nationalists." Of those, some saw violence as the only way to achieve their freedom. They would become known as "republicans."
In 1921, Britain agreed to the creation of an Irish state, but only if it kept a small section of the island known as Ulster. The majority in Ulster were Protestant and considered themselves to be British--to the point that a few years earlier, they'd threatened to start a civil war at the prospect of being abandoned to the Irish. Also, Ulster's capital, Belfast, was the world's leader in shipbuilding, linen and rope making. With Her Majesty's Navy circling the globe and preeminence in international commerce at stake, the United Kingdom was not about to lose Ulster. In the new Republic, the split was controversial before it happened: Irish hero Michael Collins was assassinated for his agreement to the partition. In Ulster, it seemed like a good idea at the time. Except, of course, to Catholics who remained an oppressed minority.
But the very reasons for keeping Ulster British were the seeds of disaster.
Over time, Her Majesty did not rule colonies around the world, so she didn't have the need for a huge navy, and there were other ways to transport passengers and goods. Then, with no need for ships, there was no need for rope or linen. Suddenly, a man with 15 years in a skilled trade faced lifelong unemployment. For his sons, prospects were worse: never getting a job to begin with. The speed with which this occurred was breathtaking: Forty-year-olds remember growing up when thousands (mostly Protestants/British) worked in the shipyards. Only a few hundred, if that, still work in the yards. And since the rest of the economy had relied on shipping, the Protestant/British community was devastated economically.
On the Catholic side, they never had anything to begin with, so the loss of industry didn't hit them as hard. All they had--good Catholics that they were--were people. But that meant Catholics' already impoverished circumstances were getting worse. There were more Catholics, and they were getting angrier.
Meanwhile, the Protestants/British were terrified. The native industry was gone; the rest of the United Kingdom was propping them up economically. And nationalists wanted to unite with Ireland. Ireland--with its only export being immigrants who fled the country each year--had an economy that made Northern Ireland's look sterling by comparison. Add to that "Irish rule was Rome rule": Ireland forbade divorce, birth control and followed papal decrees to the letter. It would no doubt discriminate against Ulster Protestants; it would strip them of the little they had left. Their only hope was to be "unionist," to remain part of the United Kingdom.
Tens of thousands, poor, angry and feeling powerless. It was a powder keg just waiting to explode.
The fuse was first lit not in the Republic of Ireland but the southern United States. Throughout the 1960s, while reading newspaper want ads saying "No Catholics," Catholics in Northern Ireland watched the American civil rights movement. They said, "We have a dream, too." And they, too, took to the streets.