Role in the film
King Brian lives in the Fairy Mountain, under the ruins of Knocknasheega, where he guards all of the greatest treasures of Irish history and myth. He has enjoyed a long-running rivalry/friendship with Rathcullen's local storyteller, Darby O'Gill. The two frequently try to outwit each other as Darby tries to win the crock of gold from Brian. His closest attempt was foiled when Brian tricked him into wishing a fourth wish, which undid the first three. But King Brian has a special fondness for Darby.
When he learned Darby was being forced to retire, Brian brought him to his kingdom to stay. But Darby tricked him by playing a tune on a violin that inspired the Leprechauns to go on a fox hunt so Darby could escape. But Brian angrily came to Darby's house to take him back. Darby tricked him again by playing him with whisky and singing a drinking song, keeping him until morning when his powers were gone. Powerless, he was forced to grant Darby three wishes. Darby's first wish was for Brian to stay by his side until he made the next two. As a further indignity, Brian was forced to be carried in a sack wherever Darby went.
When Darby attempted to show Brian to Michael MacBride, the leprechaun king disguised himself as a rabbit, making it appear that Darby was poaching. When Darby casually told Michael 'I wish you could see him,' Brian called out "Granted!" tricking Darby out of another wish.
When Darby admitted he could not think of another wish, Brian offered to do it for him and make his daughter, Katie fall in love with Michael. Brian visits both of them in their dreams to encourage their love. The next day, before Darby can make his last with, Katie bursts in and causes the sack to fall over. Brian escapes in the form of a rabbit.
That night, after suffering a dangerous fall, Katie was injured to the point of death, and the Banshee summoned the coiste-bodhar, a spiritual coach driven by the dullahan to carry her soul to the Otherworld. Darby summoned King Brian and sacrificed himself for Katie by making his third wish to go in Katie's place and Brian granted it.
As a final gesture of friendship, Brian offers to ride part of the way with his friend. To save Darby, Brian tricks him into making a final fourth wish ("wishing" that his friend could join him in the afterlife). As before, this fourth wish negates all the previous wishes and spares Darby's life. Darby is saved and King Brian has the last laugh in their running battle of wits. Presumably, since Brian is immortal, he can survive the trip to the Otherworld.
- Jimmy O'Dea was uncredited in the film because Walt Disney hoped to create the illusion that he was using real leprechauns. He even filmed a TV spot of himself asking King Brian to appear in the film, and included an insert to the opening credits thanking King Brian and his leprechauns for their 'cooperation.' Disney even went so far as to film the Walt Disney's Wonderful World of Color (1954) episode, "I Captured the King of the Leprechauns" (#5.26), in which he and "Darby" (Albert Sharpe) manage to corner King Brian and convince him to participate in the film along with his people.
- When Michael doesn't kiss Katie, King Brian (Jimmy O'Dea) exclaims "And him a Dublin man!" O'Dea was born and raised in Dublin.