You've summed it up well. Protestants in Ireland have always been a small minority and its a miracle that they are still there. In the 1800s they came under pressure from landlords and Catholics too and many of them headed off to America.Incidentally, the Irish immigrants to the American colonies in the 1600s and 1700s were mostly Scots-Irish Protestants from Ulster. They were seeking farmland, which wasn't available in overcrowded Ulster. They were Presbyterian yeomen farmers with little attachment to the Anglican Irish ascendancy. Once in America, they supported Cromwell's Puritan government, which had overthrown the royal government. Later, they mostly supported the American Revolution. And their descendants were in the vanguard of the wave of American settlers who moved westward across the North American continent throughout the 1800s.
The Catholic Celtic Irish immigration to America didn't start until the Potato-Famine of the mid-1800s. If it hadn't been for the large Protestant Scots-Irish emigration out of Ulster in the 1600s & 1700s, they'd probably now constitute about half the population of modern Ireland.