The druids are dead, and when they died off, so too did all of their secrets because they never wrote anything down. The only reason we have a vague idea of what they did is due to outsiders witnessing/observing them and then recording down what they saw.
Anyone who claims they are a druid is a neopagan reconstruction of what people feel the druids would be like. But there is no certainty in that.
If you are interested in Irish magick, then I would suggest investigating traditional irish Witchcraft and the mythologies surrounding the culture. The druids were very highly regarded in their society. They were upper class and highly revered. The Irish witches were your everyday common folk, but Irish Witchcraft was still built around the same society, religious beliefs, and time period the druids stemmed from. In my opinion, that is the closest you'll ever come to true druidity--by taking Traditional Irish Witchcraft and placing a shamanic spin to it, as well as reading what was observed from the druids by outsiders (taking into consideration possible misinterpretation of intentions and meanings).