Two sweet folk metal bands are Eluveitie and Korpiklaani. Especially Eluveitie because they use a whole range of Celtic instruments.

<a href="http://www.youtube.com/watch?v=iijKLHCQw5o">Eluveitie - Inis Mona</a>
<a href="http://www.youtube.com/watch?v=fZbucSufNm4">Korpiklaani - Keep on Galloping</a>