what exactily was a celtic shamin responcable for doing?

Answer 1

If by Celtic shamen you mean Druids, then, in the words of Caesar:
" The Druids officiate at the worship of the gods, regulate public and private sacrifices and give rulings on all religious questions." He also said "...they act as judges in practically all disputes whether between tribes or between individuals."

If you mean Celtic shamen as in Vate (also known as Ovate), then:
The members of the Order of the Vate are the "diviners and natural philosopher," described by Strabo, of the Celtic society. Ammianus said that they "investigated the sublime and attempted to explain the secret laws of nature." Mainly, the Vates are searchers to 'occult' mysteries, and were professional seers who used divination that was inspired by dream, touch, and poetic composition. The Vates were also the doctors and healers of their society, and they had the knowledge of using herbs as medicine. In myths, the Vates are said to have developed the ability to shape-shift, as well as other shamanic powers, like traveling to the 'other world'. This travel often required a 'detachment from life,' so to say.

That help?

Answer 2

He would invoke and take on the seeming of one or another of the wild gods when the Celtic peoples in the villages nearest the wild needed to interact with them, when a group of young men went thru their rite of passage, when leaders were looking at the beginning and the rest of the year, when people were greiving and farewelling their recently deceased, and when people were celebrating the turning points of the seasons and the year.

That kind of thing.

Answer 3

I googled "Celtic shaman" and got, among others, these sites:
http://www.celticshaman.com/index2.html (modern)
http://www.faeryshaman.org/ (modern)

The following link is to a well written article that is interesting in a poetic way but a little vague about details:
http://www.blessingscornucopia.com/Celts_Celtic_Ancient_Civilization_Shamanism_of_Ancient_Celts.htm

Hope that helps!