What do you consider ironic about two groups of long-isolated (from each other) humans making contact and there being conflict?

There are no similar events at this scale to compare to (what is your context for irony?).

Looking at the smaller-scale examples, the presence of conflict seems entirely consistent, and hardly ironic.

I’m guessing you’re reading “brought together” as some sort of idiom for happiness and mutual understanding. That would be an incorrect reading, in which you are adding your own meaning and assumptions to what was written. Take it more literally.