When it comes to traditional Irish music (and Scottish and Northumbrian too), playing from dots doesn't work. If you have 500 or 1000 tunes under your belt already that you've learned by ear, then maybe, just maybe, you can fast-track the learning of a new tune from dots. Otherwise, it's anathema. There will be those of us here (me included) who have sat with pretty decent musicians used to playing music from other genres requiring a more formal approach who try to play "our stuff" from dots who simply don't get it. As the estimable Alan Ng said on his website (forgive a possible slight misquote), learn it by ear and learn it right.

As for Ashokan Farewell, it's not a traditional tune but that doesn't mean you can't meddle around with it if you're playing the melody on your own. But that isn't exactly the folk process at work. Every time I've heard it played by a bunch of people trying to do their own thing with it, it's been a bit of a mess. Maybe in a hundred years' time it'll evolve nicely. I doubt it.