Celtic Priesthood
Originally written by Dom of D20 / D&D 3e Netbooks and Downloads.

Druid means at the same time “man of the oak tree”, but also “thick with knowledge”. In fact the word druid is used for all members of the Celtic priesthood who belong to one of the three branches, each of them having its own specialization (and is represented by a specific character class). The three druidic branches are:
1) Druid.
A druid is a member of the learned class among the ancient Celts. They acted as priests, teachers, judges, natural philosophers, and sages. They wear the white linen robe and the oak crown. The druids conduct the religious ceremonies that maintain balance between The Otherworld and the mortal realms of humans. They otherwise preside over the structure and order of the Celtic society, and act as arbiters, judges, etc., among the Celts. While Celtic communities are ruled by kings (who normally are noble warriors), in practice these kings do nothing without the druid’s advice and agreement.
2) Fili (plural Filidh). [See new class sometimes also use the cleric.]
They were also magicians, as Irish magic is intrinsically connected to poetry, and the satire of a gifted poet was a serious curse upon the one being satirized. To run afoul of a poet was a dangerous thing indeed to a people who valued reputation and honor more than life itself.
Filidh are visionary poets, associated with lorekeeping, versecraft, and the memorization of vast numbers of poems. They are seers, healers, and various specialists such as judges or technicians. They wear the green robe. While filidh are bound to show the normal reverence to all the Celtic gods, they nonetheless must choose one as their patron deity. That is, the god who exemplifies their field of study. For example, a fili specialized in medicine will choose Diancecht as his patron deity; and one specializing in magic will choose Math Mathonwy. In any case, filidh help their communities with their knowledge and magic. In the absence of a written culture, they are the repositories of sciences and techniques of the Celts.
3) Bard. [Use the SRD bard class; however, the description / ethos is different.]
Bards are poets, musicians, teachers, historians. The Celtic poets, of whatever grade, were composers of eulogy and satire, and a chief duty was that of composing and reciting verses on heroes and their deeds, and memorizing the genealogies of their patrons. It was essential to their livelihood that they increase the fame of their patrons, via tales, poems and songs. They wear the blue robe. They are the keepers of history, and praise or blame nobles, passing on the memory of tribes and heroes in an exclusively oral culture. Bards also support their tribe and patrons with their magic, but otherwise do not perform religious ceremonies. Many bards are travelers going from tribes to tribes, making a living at praising those who would host them, but also bringing news from the outside world.