Nobody who plays Scottish, Irish or Breton traditional music seriously would refer to them as "Celtic". That word is marketing hype with racist overtones.
I play Scottish and Breton regularly, Irish less systematically though some every week, on a mixture of instruments. For Scottish music you want a G-pitch ocarina. A 10-hole transverse like the Menaglio is ideal but the MO in G would be ok. A C-pitch ocarina is absolutely no use whatever for that. On the other hand, it will work for some Breton tunes (you will also need a G).
For Irish music, a D-pitch instrument is your best bet - Hickman, Rotter and Posch make those. But Irish music tends to be flute- or violin-based, or elaborately vocal, and hence often has a range too wide for any single chamber ocarina. Normally I reach for a recorder; I can't think at the moment of any Irish tune I would play on the ocarina for preference, except for the reel "The Gravel Walk", which has a range more typical of Scottish tunes and goes on an ocarina in G.
(that's about the speed I use)
Avoid subholes. You will never need them in traditional music and they just weaken the sound of the instrument in places where it needs to be accurate and assertive. The MO has one, which is one too many but it's less damaging than in some other designs.