26 treble is all one usually needs for the Celtic genres, as with Tex-Mex (unlike klezmer/Balkan, where you really need that low b-flat and low "a").
The small % of Celtic tunes that go below middle-C don't linger there and few tunes indeed would not be easily rearrangeable. A good Hohner or Weltmeister is fab for Irish/Scottish dance tunes. And if one wanted to go premium, the beauty is that the Italian makers have tumbled to the demand for quality small boxes and are offering them. However, the Hohner has a sentimental place in dance-based roots/folk music in many traditions including Scotland and Ireland, and I suspect Sandy B is making a point as well as making fine music.
In Ireland, Michael Sheehy is a wonderful exponent of the polkas, slides, and slip jigs of the County Kerry/Sliabh Luachra region. I've seen him pictured/filmed playing a small Hohner-ish PA that appears to be a Galota, which is essentially a Weltmeister. Having trouble finding clips I've seen in the past where he's viewed front-on so you can see the PA, but here is a wonderful example of his polka playing:
