tones
Tones deaf
Presumably, all Ireland was Catholic before the influx of Scottish immigrants, but it seemed that the Ulster troubles only came to the fore after 1921; a fair bit after, too. My history is hazy on this, though religious sectarianism often seems to manifest itself in nationalism.
Correct. It was James I (James VI of Scotland) who sought to change the character of Ulster by means of the Plantation, an early exercise in ethnic cleansing, which didn't quite work out (the implants needed the natives, and the result was a peculiar mixture). James actually wanted people of the Established Religion, not the Calvinist Scots, who came anyway, and for a time were as persecuted as the Catholics. They used to sail to Scotland on Sundays where they could legally hear a Church of Scotland sermon! Ironically many of the early heroes of the fight for Irish independence, such as Wolfe Tone, were Protestants.
The mass emigration to the USA in the wake of the potato famine led to the rise of the Fenian Movement in the USA, the direct forerunner of the IRA. Eamon de Valera, the only IRB commandant to escape execution in the aftermath of the 1916 rising was born in the USA (one reason why he escaped execution).
An outstanding account of the Plantation is Jonathan Bardon's "The Plantation of Ulster".