The Druids were the priestly caste of ancient Celtic society. The original Druidic faith was polytheistic, whilst also venerating natural phenomena such as the sun, moon, and stars, as well oak trees, mistletoe, holly, streams, lakes and hilltops. As well as their role as priests the Druids also provided service as arbitrators, healers, scholars and magistrates. Typically the Druids have been represented as being all male, but the lack of any real evidence either way means that this may be a later distortion of the truth.
The trouble with really understanding the role of women in Druidism lies in the fact that Celtic practices were modified, firstly through the Roman invasion of Britain and Gaul and then later during the Christianization of their lands. The Christians brutally suppressed the Druidic belief systems and attempted to destroy all records of Druidic beliefs, practices, and imagery, or at the very least attempted to subsume these beliefs, practices, and imagery by recontextualizing them with Christian meanings.
This means that to get at the original role of women in Druidic society requires stripping away the later modifications and destruction of the culture and reconstructing the details of the original society. This involves the use of scholarly endeavors through academic branches such as history, linguistics, archeology, and anthropology to provide an authentic account of the roles of women in the society.