Kevin Hearne is a whimsical writer who likes puns. He is a joy to read. He creates a complex world with supernatural beings and gods of many pantheons, and odd characters, quasi-humans like himself who live in the interstices. The protagonist Atticus O'Sullivan (Siodhachan O Suileabhain) is a druid. Actually he is the last druid. He is 2000 years old because he drinks his special tea Immortali-tea. He has a dog that talks to him psychically and says the funniest things.

This is urban fantasy but with some differences. There are the usual vampires and werewolves and witches but in general the main focus of the stories are druid and gods. There are various pantheons of gods in these stories, from of course, the Irish pantheon, but as well: Norse, Greek, Roman, Russian, Amerindian, Hindu and even Christian, they are often the main antagonists. The gods of different pantheons can fight each other or even team up on occasions. There are different pantheon related planes, heavens and hells.

The magic is kind of consistent and follows rules, at least druid magic does and the other characters have different kinds of magic that follows rules that you might not quite understand. Druids were created by the Irish gods to protect the earth. Although that doesn't necessarily mean what you might think. Anyone can become a druid or a witch but the druid training takes years and up till the first book, none of his apprentices survived to become a druid.

The Iron Druid entry in tvtropes is fascinating but lots of spoilers. It is to be enjoyed when you've read all the books: 8 novels, three novellas and four short stories.