Ah yes. Whataboutism. Suppose a robber acts in defense of a person about to be robbed. That may or may not make them a hypocrite, but it certainly doesn't make them wrong.
Or would you say it would somehow be more right for the robber to stand back and allow the robbery "because they're in no position to point fingers"?
If anything it would be more a 'tu quoque' fallacy than whataboutism, because the latter tries to shift the attention to an unrelated topic, whereas here it is occupying land both times.
It certainly weakens the criticism, because the robber in your example might do the right thing, but if they really opposed robbing, surely they wouldn't do it themselves? As you said, it makes them a hypocrite, and makes you question their motive for measuring two cases with a different yardstick.
It's a really complicated situation as far as my understanding goes (I'm British for context). I believe most British would support Irish unification, at least from the people I speak to. But still the majority (or close to) would vote to remain in the UK. Plus it'll be outrageously expensive a transition for Ireland. I don't know why this is and would like to know.
It's not an easy situation, and in my own personal opinion we should be uniting as friends, allies and equals.
As a UK person, I look at Japan as a nation very similar to ours, they've successfully united 4 islands (plus many small) much larger than the UK and I'd love to understand how it seems so easy. They had just as many conquering bastards, but everyone is happy being Japanese.