Protestants have always played a huge part in Irish nationalism - assuming that 'protestant' means dissenter as well as Church of Ireland.

Most of the 18th-century revolutionaries - people such as Theobald Wolfe Tone, Henry Joy McCracken, I think Robert Emmet, etc were all Protestant.

I'm not certain of this, but think the Young Irelanders were primarily Protestant. In the lead-up to 1916, the Gaelic League was largely Protestant, leading to a distrust for the Irish language among the Catholic priests and bishops that has persisted to this day; the same people were nationalist, but tended to be in the branch of the Volunteers who followed Redmond into World War I to fight in the British Army (often to return to Ireland afterwards to fight for Ireland in the War of Independence).

From the time of the Reformation, it became legally disadvantageous to be Catholic. Many families therefore educated the most promising farmer or manager as a Protestant, and the branch of the family that 'jumped' (converted to Protestantism) was the one that ended up with the land. The Catholic side drifted into poverty or emigrated, initially to the southwest of France, later to North and South America, finally to England, Canada and the east coast of North America.

Once Ireland had won independence, the position was reversed in the first few years of the Irish nation, and Protestants sold up and flooded out of the country in the same way that the whites flooded out of Rhodesia in the first days of Zimbabwe.

Now the Protestant churches are opening again with an influx of Protestants from Africa, and a welcome reopening of Protestant schools is happening. These people are not currently full of national feeling, but inevitably their children and grandchildren will be.