Currently Ireland has a political division between the Republic of Ireland and Northern Ireland.

I don't really know and we probably can't fully predict exactly what the procedures leading to this would be, but colloquially "it's one sovereign state" is the end condition we would be looking for. Something that looks an awful lot like unification but has some silly caveat, will probably still count as unification. If there are news headlines like "Ireland is now united", that's the kind of thing we're looking for.

It has to be a major change, not just a tiny shift.

If it's not clear which decade applies ("they voted in 2099 but it didn't take effect until 2101"), this question should lean toward the "when did it actually go into effect" side.

Please help me if there are important things in the criteria that I'm missing. I'm interested in when these kinds of events will happen but I don't know enough to be an expert on every one.