Eoghan Ó Cinnéide, Anglicized as Owen Kennedy, is a major character in the later books of the Iron Druid Chronicles, though he is spoken of in earlier books. He is Atticus O'Sullivan's old archdruid from the Iron Age, the very person who trained him to be a druid. He becomes a POV character from Shattered onward.
In Hunted, The Morrigan gives Atticus the location of a specific Time Isle before her death, which he finally visits during the epilogue. He is surprised to find his old archdruid there, frozen in time on the island after having been placed there by the Morrigan 2000 years ago. After being released, Eoghan is immediately out of place, knowing nothing of the modern world or even the English language. Atticus sets about teaching him how the world has changed and about the events that have transpired up until that point. He also gives him some of his Immortali-Tea, restoring his youth to about his mid 40s, as he felt making him the same age would make things awkward. He takes the name Owen Kennedy as his modern name.
Owen, despite his wisdom, is frequently shown to be ill-tempered and rather crude, often using profanity and speaking bluntly about bodily functions. He is highly critical of Atticus at times, both in the past in the Iron Age and in modern times, often accusing him of "cocking things up." He later bonds over this criticism with Greta, a werewolf who's pack formerly served as Atticus's law firm, and the two enter into a romantic relationship. In Staked, Owen starts a grove and begins training a group of young children from all over the world to be druids.
Owen's preferred method of fighting is using his fists, which is later enhanced when he receives a pair of shape-shifting brass knuckles from the Celtic smith god, Creidhne. His animal forms are a black bear, a ram, a red kite and a walrus. His favorite is the bear, while he seems somewhat ashamed of his walrus form.
Like Oberon, Owen has his own twitter account (operated by Hearne), where he frequently makes humorous remarks about the modern world and trades barbs with the wolfhound.
Owen's Twitter (His profile pic is actually Tom Hardy)