@Zuider, I am very familiar with Scala! In fact, the scales contained in the Tune.js archive come from the Max Magic Microtuner .mtx archive, which was compiled from the Scala .scl archive. It seems the .scl archive has surpassed the .mtx one, which I think was last updated by Victor Cerullo in 2010. I'll have to try to integrate the new additions to Scala into the tune.js library. I'd also like to add functionality where you can define your own scale. Maybe for version 2.0!
As to your point about pitch variation in microtonal music, that is often the case in a lot of traditional musics that make use of microtones, for sure. It is part of the character of a lot of that music, and some of the Irish music that you mention, to have a lot of subtle variation and microtonal embellishments. I read somewhere that non-fixed pitch instruments, such as violins or the voice, tend toward pure intervals naturally.
But there is a lot of music on fixed pitch microtonal instruments, such as the music and instruments of Harry Partch, La Monte Young's Well Tuned Piano, and refretted guitars.
Lastly, you could definitely program the kind of variation and fluidity into the web audio instruments that you use with tune.js. The demo just uses a piano sample for demonstration, and to have long sustain to hear the microtonal harmonies. I would love to hear this used with more portamento or with a pitch bend function, to get more natural variation.
Thanks for sharing the Irish music links, I'll definitely take a look!