Usually works for me.....

Are the Protestants descriminated against in Ireland?
Not much idea. Based on anecdotal evidence, possibly.

Are the Catholics descriminated against in Ulster?
Yes, but more so in the past than now.

Are the Ulster Prostestants afraid of being treated like equals, or of having done unto them as they have done unto others?
Or it could just be that they were born British and want to remain British.

One thing they might try to remember: if Ulster joins the rest of Ireland, they will be joining a nation that has lived at relative peace and increasing prosperity since severing their ties with the Empire. The reasonableness bred of peace and prosperity stands a good chance of diluting the impulse toward vengeance.

You may not have noticed but the UK has also lived in relative peace and prosperity since it lost the Empire too.
You may not have noticed but Ulster has lived in relative peace since the Good Friday accords. Power sharing is still unproven, but any arrangement that can elect both Gerry Adams and Iain Paisley and get them to sit together can't be all bad.
Now if we can only convince them to stop voting for the likes of Paisley and Adams maybe they'll all get along a bit better.
Vengeance? So far your argument seems pretty one-sided as far a vengeance goes.
I'll be the first to admit that British rule of just about anywhere has been abysmal, racist and often genocidal. For those very reasons we no longer have an Empire. Injustices against Catholics in NI abound in fairly recent history.
But we got better. We lost the Empire. We sat down (largely at the behest of the US) with the terrorists.
We talked. We came to an agreement with Eire. We got the moderates on both sides to come to an agreement. We put in place a power-sharing agreement that left both integration and partition on the table for the future. We changed the RUC. Things are getting better for all the people in Ulster.
The Irish/NI Catholic extremists haven't exactly been blameless victims either in the last 30-40 years. Don't forget that. The 1980s are more relevant to the Brits than 1607, the Battle of the Boyne, the Famine or even Bloody Sunday........