Read all 20 pages and theres a few Irish Folk metal bands not mentioned that might pique some interest(as you can guess from my name, I'm Irish)
Mael Mordha Celtic doom metal(self proclaimed)
http://www.youtube.com/watch?v=C5WNK4EG55A Cluain Tarbh
Waylander Death/Black Folk Metal (Depending on album really)
http://www.youtube.com/watch?v=wluU1kX-cfA As the Deities Clash
http://www.youtube.com/watch?v=L06lbxs5xGw King of the Faeries
Cruachan(one person mentioned them already!) Black Folk Metal
http://www.youtube.com/watch?v=PGhxfia-lEI Cuculainn
Primordial Celtic Metal
http://www.youtube.com/watch?v=oVfPr39HqQo Gods to the Godless
I would recommend these bands to anyone interested in folk/death
As a comparison, some similar bands I would listen to are
Turisas
Tyr
Ensiferum
Korpiklanni
Finntroll
Skyclad
Burzum
Eluveitie
Now, getting away from Folk-y types.
These bands arent Irish, but nobody mentioned them!
Destroyer 666 Death Metal (Eh, well, kinda, maybe Battle-Death or something)
http://www.youtube.com/watch?v=Ax7cdHiZ010 I am the Wargod
Ministry You could argue Industrial, but their last 3 albums were solid Industrial Metal
http://www.youtube.com/watch?v=CBGMW86u1Qk No W
Neurosis Genre: N\A (Listen to more than one album and you'll understand, I believe the term for their genre is Cult of NeurIsis, comprised of Cult of Luna, Neurosis and Isis. Though tbh, I really dont think the others compare to Neurosis, but thats my opinion. And this selection may annoy some genre-elitists
)
http://www.youtube.com/watch?v=fmdmnnv2NkY Locust Star
Other misc stuff I would listen to
Mastodon
Celtic frost
Slayer (duh)
Altar of Plagues
Electric Wizard
Strapping Young Lad (and Devins 'Ziltoid the Omniscient')
Isis
Moss
Cannibal Corpse
The Berzerker
and more that I cant remember right now.