There are pieces of Manx music and song available commercially, including by some of the people already mentioned. However, in addition, I can heartily recommend pieces by Caarjyn Cooidjagh, a Manx Gaelic folk choir (the name means 'Friends Together'), and The Mollag Band (a mollag is a tarred dogskin float - don't ask!), a sort of contemporary take on traditional music, with modern-day political pieces inter-meshed with tunes with a Manx provenance.I've use the term 'Manx' - it does seem to lead to some misunderstandings! It's the adjective referring to the Isle of Man, between Ireland and England to the west and east, and between Wales and Scotland to the south and north.
I'd also point out that most people seem obsessed with Irish (particularly) with a smattering of Scottish and an acknowledgement towards Brittany (there's a whole lot more than Stivell there).
There is some great Welsh stuff, and some fine stuff coming out of Cornwall. If you want to be argumentative about it, you could also inclued Galician (not accepted by the Celtic League or the Pan-Celtic Festival, but very much a part of the Festival Interceltique in Lorient: chielfly through Carlos Nunez and groups like Milladoiro).
Shoh slaynt,
Bobby Bob