I wonder how many of these editors hang out with harpists. (I know a harper who has five harps.)

Nothing says that O'Carolan had only one harp. He could have had a gut-strung and a brass-strung. He could have had one and changed the strings from time to time. As for tunings, he could have tuned the same one different ways at different times. To play music "that doesn't stay in one key" you can simply select the crucial string and push on it with your finger until it's sharp.

Not all harps are as big a refrigerators. It wouldn't have been much of a problem to transport two (or more) small-to-mediums in the pony trap he is said to have had.

Carolan was a musical man, and he played for years and years. He was blind, and he didn't have much to do expect play his instrument. With someone like that, it isn't logical to wonder if he improvised, if he added harmony, if he tuned different ways. That's like wondering if a cat meowed. The trick would have been to STOP him from doing all the above.

However, anybody who's waiting for written documentation of all the above will probably have to wait forever. So don't wait for permission to play his things and enjoy them. Just be your musical self and do it.