Since Druids committed very little to written forms until after the introduction of Christianity, there are few, if any, first-hand accounts by Celts themselves. Only the myths that have been transmitted through the accounts of the Romans and Christian monks have survived. Legend has mystified many of the female rulers of ancient Celtic society; giving them mystical powers and making their lives seem too extraordinary to seem true. As a result it is very difficult for people today to know if these women truly did have such powers or if they were indeed Druidesses. Women such as Boudica, Onomaris, and other nameless rulers/Druidesses whose burial tombs were found at Vix and Reinham show that Celtic women, in some instances, may have wielded power as much as men, but the evidence remains difficult to decipher.
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In other instances, however, the only reference to women with great power is through the term sorcereress. Fedelma, a "woman from the Fairy, or the Otherworld" (Green, p.102) was a part of the mystical Queen Medb of Connacht’s court. "Fedelma first appeared to Medb as a beautiful young girl, armed and riding in a chariot" (Green, p.102) wearing a red embroidered tunic, sandals with gold clasps and a "speckled cloak." She informed the Queen that she had studied poetry and prophecy in Alba, "a supernatural land belonging to Scáthach" and then warned her of the advances of Cú Chulainn. Medb then asked the girl if she had the power of ‘sight,’ Fedelma affirmed this and told Medb the chilling prophecy of her troops "I see crimson, I see it red." Her prophecy came true, Medb lost the battle and Cú Chulainn perished.

Other tales of the Druidesses that have survived often include the subject of sacrifice. "They were grey with age, and wore white tunics and over these, cloaks of finest linens and girdles of bronze. Their feet were bare. These women would enter the [army] camp, sword in hand and go up to the prisoners, crown them, and then lead them up to a bronze vessel. . . One woman would mount a step and, leaning over the cauldron, cut the throat of a prisoner [of war], who was held over the vessel’s rim. Others cut open the body and, after inspecting the entrails, would foretell victory for their countrymen" (Green, p.97).

Druids had many responsiblities, but their main duty, especially with the centralization of Celtic society, became to advise Kings and Queens. Dreams and prophecies were questioned by royalty for their significance and they interpreted events in various kingdoms. As a result, the power of the Druids and Druidesses was very great for not only were they the sole priests of Celtic religion, but they also held great sway in political matters.

http://www.unc.edu/celtic/catalogue/femdruids/index.html

Witches: The Legacy of the Druidesses
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With the spread of Christianity, Christians dubbed female Druids as witches in order to make their power seem evil such as the witches of ancient times to the native Celtic people. The Christians feared the Druids and Druidesses not only because they were a source of great religious power and the center of knowledge for their people, but also many of them preached strongly against conversion to Christianity (Ellis, The Druids, p.72). Consequently, the Christians began to write down information, which put Druids and Druidesses in a bad light and helped perpetuate their ultimate downfall.

In Medieval times Christians altered stories Druidess heroines to evil witches. In this manner, the Christians effectively began to sway the opinion of the clergy and the people that knowledgeable women and the Druid order itself were wicked. Such stories such as Dahud-Ahes, daughter of the six century king Kernev, whom Ellis says was "undoubtedly a Druidess adhering to the old religion, [and] who [was] then transformed into a sorceress by Christian scribes" (Ellis, The Druids, p.104). She was strongly opposed to Christianity, and therefore her city of Ker-Ys was destroyed by a flood and she was turned into a mermaid "as she sinks beneath the waves, proving that [St. Guénolé's] magic is just as good as any Druid['s]" (Ellis, The Druids, p.104). Professor Markle states:

Apart from representing paganism in opposition to Christianity, however, she [Dahud-Ahes] also symbolizes the rebellion against masculine authority...The full significance of this act becomes clear when one considers her dissolute life as contrary to the teachings of the Christian Church, here represented by St Gwénnolé, himself the very symbol of masculine authority (Ellis, The Druids, p.104).

http://www.unc.edu/celtic/catalogue/femdruids/Witches.html

Boudica

Boudica, a legendary figure in British history, played a large part in resisting the power of the Roman Empire. In 60 AD soon after the arrival of Roman armies, Boudica then thirty-five years old, was the widow of Prasutagus and ruler of the kingdom of Iceni (Green, p.95). Her husband had entered into a contract with the emperor, Nero that stated that her could preserve his kingdom if he kept his kingdom peaceful to act as a "buffer state" (Green, p.95). Boudica’s assumption of power after his death angered Roman officials for it was "illegal in their eyes" for a woman to rule a kingdom. As a result her kingdom was "ravaged and looted," and her condemnation of these acts resulted in an unclad public wiping and the subsequent rapping of her daughters which she was witness (Ellis, Celtic Women, p.87).

At this point Boudica emerges as an absolute ruler of the Iceni and gathers the loyalty of the surrounding kingdoms. According to Tacitus "this is not the first time that Britons have been led to Battle by a woman" (Ellis, Celtic Women, p.87). "Boudica shoed her prowess as a military strategist" (Ellis, Celtic Women, p.87) and annihilated a large Roman legion. They went on to destroy Londinium (London), Camulodunum (Chester) and Verulamium (St Albans) and outrage and humiliate sexist Roman officials. However, her campaign did not succeed in freeing the whole of Briton from Roman forces for she was defeated at a trading post called Londo. Some scholars speculate that she committed suicide soon afterwards for texts do not mention her after this time.

According to Dio Cassius, Boudica was a priestess of the goddess Andrasta who is describes as the goddess of victory (Ellis, The Druids, p.92). As a result Ellis states that "an argument could, therefore, be made that Boudica was a Druidess as well as a Queen" (Ellis, The Druids, p.92). Ellis further argues that "as Druids were merely the intellegensia of ancient Celtic society, equivalent to the Brahmins of Indian society, . . . a Druid could become a king or Queen but a king or queen [could] not necessarily become a Druid" (Ellis, Celtic Women, p.93). This argument validates the suspicions of many scholars who speculate the position of such women as the 'Vix princess,' the 'Reinham princess,' and other women Celtic rulers.

http://www.unc.edu/celtic/catalogue/femdruids/boudica.html