There may well have been women druids; personally, I think there almost certainly were bandrui though they were not in the majority. The evidence we have for their existence consists of references to them in the myths, and in a few Classical texts.
There are references to bandrui in the medieval Irish tales. Conchobor Mac Nessa’s mother Nessa was a druid, perhaps one reason he took his name from her, rather than from his father. Finn was raised by a female druid. Scathach is explicitly called both a flaith, “prophetess,” and a druid and she prophesies about Cú Chulainn. And of course there are the banflaith (sometimes banfili), the “women poets,” most notably Fedelm in the Táin (though one may argue that flaith and fili are entirely separate, it is a distinction that is often difficult to discern in the medieval texts themselves).
I suggest those who are interested check the index in either Piggot’s The Druids or Ellis’ The Druids; Ellis is perhaps more interested in the question of women as druids than Piggot, but he is less reliable. There are also some pertinent observations in L. Allason-Jones. Women in Roman Britain. London: British Museum Press, 1989. 146-151. Also see H. D. Rankin Celts and the Classical World. London: Croon Helm, 1987. 252-55, and ff. Rankin discusses the anecdotes from Scriptores Histories Augustae (fourth century C. E.) regarding Roman emperors who consulted female druids. Although Rankin provides a reasonably thorough summary of the evidence for women who were druids, he tends to think it less likely than many of his colleagues do. There are other anecdotal Roman bits of evidence, like that of Flavius Vopiscus, who discusses encounters between Diocletian and others with female druids. Last but not least, Tacitus’ discussion of Veleda in Hist. IV; the name Veleda is cognate with Irish flaith.