Articles

Where to find in story mode:
Land 2, Land 3

Buildings:
Buildings of the Celtic are mediocre, some of their cheapest huts are very small (which is good for placing many buildings), but their biggest houses are not so cost/size effective as Norse or Indian ones.

Wonder bonuses:
Storm Miracle ("Increased" and "Extreme" versions deal more damage. No difference in price/effect was noted about the normal Storm Miracle, but the raindrops and thunder sound disappear. Weird!)
Forest Miracle (trees give a lot more wood)

Opinion:
It really depends on the map. If you have access to Forest Miracle, then the Celtic Wonder becomes extremely useful, providing you with lots and lots of wood (maybe even more than Wood Miracle increased by the Norse). And more resources means more wonders, more wonders means even more resources and influence area.
Same for the Storm: if you have Increased or Extreme versions, then Celtic Wonder can turn them into one of the most devastating weapons in the game (a bit hard to control, though).
Without those Miracles Celtic Wonder becomes nearly useless. It is also very big, really hard to find a building place for it.
P.S. Weird, but Celtic music is also being used by the Norse and African, which do not have own one. Why Celtic? Maybe it was the very first Tribe to be made.

Celtic Tribe Music: