Every Druid is a Bard, but not every Bard is a Druid.

The reason I say this is because a Bard's training lasted 7 years, a Vates, 12 years while a Druid continued on in their studies for 20 years.

Each druid would their own specialty but they've all attended a college where they've undergone at least 12 years of extensive training which includes all the disciplines before branching out in their specialty such as Brehon (law) or Cainte (magic) or Fili (Bardic training). While a Ollamh Cainte would know more of the ways of magic than a Ollamh Brehon, a Brehon would still know magic as a Cainte would still know law.

After Christianity began to take hold in Britain and Ireland, the Druids found themselves being phased out (remember that in Britain, the Romans had already seriously damaged the Druids). The Brehons were the longest lasting branch of the Druidic teachings, lasting until the early 1600s when the Brehon Law in Ireland was replaced with English Common Law.

I recently read a very interesting theory about the role of Druids in Celtic society. Sean B. Dunham in an article Caesar's perception of Gallic social structures puts forth the theory that we may have the role of the Druid all wrong. Whereas the modern view of them is that of priests, that in actuality they had the tripartite duties of priests, philosophers and judges. The ancient world didn't have the separation of church and state that we do now. Instead of being a mysterious magician priests, they actually were an oligarchical elite with both judicial and religious duties.