I just looked up Kec. Gunung Meriah, Singkil. (Just because the data are fairly complete).
They classify ethnically as Dairi, Aceh, Minang, Jawa, and Lainnya.
It is not clear what they mean by 'Dairi', but it may mean both Pakpak (likely to be Christian) and Singkil (all Muslim), since they...