Just as I thought, the key signatures are all over the shop. It doesn't even match up to the harpers playing the tunes. (I only looked at O'Carolan tunes.)

I didn't analyse how close the different keys are for each harper's tunes, in the sense of how many strings would need to be retuned to go from one key to another. I might need some more caffeine before I even think about that.

My suspicion is, as I suggested before, that for pianoforte players they would need to learn all the different key signatures, i.e. become competent in using the white & black keys in different combinations, so a book of tunes for pianoforte would use a lot of key signatures. However, on a harp it is tuned to one key signature at any one time, and the only other main consideration would be accommodating the range of the tune notes into the range of the harp strings available.