Celtic Metal is a sub-genre of 
Black Metal.  Its focus is on 
Celtic Mythology and Instruments mixed with Black Metal.  Celtic Metal has developed a huge underground scene in 
Ireland, where pioneers like 
Cruachan[?], 
Geasa, 
Waylander and other such bands play.  By far the most successful album is Cruachan's "Tuatha Na Gael" which sold out its label's supplies within a few months. Having said that, it cannot be considered a great success because it was a small label and only pressed a few thousand CDs.
Sub Genres
Although still a sub-genre by itself, Celtic Metal has already spawned a few mixtures and sub-genres.
 
References
 
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