The autoharp is just pushing buttons and strumming right?

Yes, tho you also can hammer-arpeggiate chords to accompany a fiddle tune, but I am not sure how fist-shaped the hammering hand has to be to hold and direct the hammer. On an autoharp the best hammer is a long-handled iced-tea spoon or a wooden cooking spoon. (Can't really hammer melody, but can have a good time in a Celtic mix.) The chord structures tend to vary very little, so it can get boring after a while.

But you might like the plucked psaltery. With a pickup added you'd get volume, and without, it would be so quiet that you could jam with folks without being loud enough to distract from the tune being played.

How you would do that is to pluck, first times thru the tune, just the accented notes in each measure. (Build a sort of skeleton for the tune.) As speed comes with these accented notes, you'd start to sound the intervening notes in the measure. When a tune gets really familiar, then, with practice or continued jamming, the next step would be to add ornamentation.

A lot of transcribed fiddle tunes are actually, mostly, one particular tune-player's ornamentations upon tunes which, themselves, are much, much simpler. You can see this if you look at a line of Carolan music and then hear it performed-- "Where did all THOSE notes come from???"

~S~