Celtic music -- very, very big topic. I recommend starting with new-age fusion by performers with good trad credentials, moving to full-blown traditionals when you're more acquainted with the style.
Good starting points:
Song of the Irish Whistle by Joanie Madden -- very fresh and inventive arrangments, making good use of studio techniques and "found sounds". Avoid volume 2.
A Winter Crossing by James Galway and Phil Coulter -- the story of Atlantic Crossing narrated through music. Heart-winning melodies, nice orchestration.
The Leaving of Ireland by Jon Mark and David Anthony Clark -- another set that recounts the Atalantic Crossing. Uplifting and somewhat"cinematic" in its use of narration and songs.
Skyedance by Alasdair Fraser and Paul Machlis -- now we are moving into traditional pastures. Fraser is a fiddler who is at home with both folk and classical music, and it shows. Sparkling arrangment of traditional Scottish tunes backed by Machlis' piano and synth works.
For middle-eastern (Turkish, Iranian) and Central Asian (Armenian, etc) music,
Traditional Crossroads is a label that has never failed me, in terms of sonics and performance. They have a web page at
www.traditionalcrossroads.com.
My favourite: Djivan Gasparyan (leading exponent of the Armenian reed called duduk -- this is music that makes you weep)
Night Silence Desert by M.R. Shajanian and K. Kahlor -- this is a beautifully written suite for Iranian instruments and male vocalist.
Over the Wind by Ardavan Kamkar -- winning solo works for Iranian santoor.
The French label Ocora is another rich resource for this sort of music, and they have recently launched a budget series. This label is more, well, academically orientated though.