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You can't really base it on anything, as far as I can tell: any generalization brings a counter example.
You can, analysis of keys, structure and all that sort of thing have been done, based on various collections. Breathnach collected well over 10.000 tunes in his time, you can do statistically significant things from there.
The pitch of baroque flutes has no bearing on traditional music. They weren't used for that music.
I also think you should distinguish between the bardic tradition of the harpers and the tradition we have today. They're different things. The harping tradition went extinct. There's no dispute about that. You can examine keys used by Bunting but it is known he did a fair bit of changing there before publication.
The wirestrung harp's tuning/pitch can vary considerably with temperature and environmental factors. I know a very fine player of the wirestrung and he, when playing solo, more or less lets it fall where it lands, as long as the harp is in tune with itself. I imagine it would have been the same with the old harpers, no standard pitch, as keeping a fixed pitch is quite an undertaking. And the copper wire they used at the time would have been stretchier than the steel wire used today. But perhaps that touches more, again, on issues of pitch rather than key.
I have a copy of the Neal collection of 1724 here, a quick look shows there's a mix of tunes in D, G C,F and Bb. But again, I don't know what we can read from that and how that relates to traditional (dance) music, its more music for the drawing room than the music of the country houses. And I don't think the two mixed much before O'Riada started toying with both.