O'Reilly the Leprechaun is a very small green leprechaun. He has shaggy eyebrows and a beard and is dressed all in green with a green coatee (short jacket) and waistcoat, a bowler hat or canvas sailor's hat with a four-leaf clover in the hatband, short green trousers and light- and dark-green-banded stockings, and dark green shoes with gold buckles. He always carries a white clay tobbaco pipe in his teeth to complete the stereotype. There is a possible parody of him who is O'Really from Moshi Monsters.
He is seen as Ralph's imaginary friend who tells him to burn things. He is also seen during Gil, Kirk Van Houten, and the Crazy Cat Lady's St. Patrick's Day Song. He was also seen in the parade, he attacks Homer & Bart, and the well in where he is featured alongside his orange (Northern Irish) doppelgänger.
When the town was divided over whether or not the Simpsons should share the money from a painting they bought from the Van Houtens, he was for them to keep the money to themselves.
The Leprechaun allied with Sideshow Bob, Kang, and the ghost of Frank Grimes in an attempt at killing the Simpson family in revenge at a Christmas tree lot (being the fourth member that was alluded to). However, he insisted on them dancing before commencing the revenge, which ultimately resulted in Maggie Simpson killing them via beheading them (with only Frank Grimes "surviving" due to his already being dead).
- Episode – "This Little Wiggy"
- THOH – "Treehouse of Horror XII"
- Episode – "Mommie Beerest"
- Episode – "Homer and Ned's Hail Mary Pass" (Woman wearing Leprechaun suit)
- Episode – "Kill Gil: Vols. 1 & 2"
- Episode – "You Kent Always Say What You Want"
- Episode – "All About Lisa"
- Episode – "Sex, Pies, and Idiot Scrapes"
- Episode – "Them, Robot" (Seen in couch gag)
- Episode – "The War of Art"
- Episode – "Singin' In The Lane" (non-speaking cameo)
- THOH – "Treehouse of Horror XXVI" (Wanted: Dead, Then Alive)
- THOH – "Treehouse of Horror XXVII" (Intro)
- Video game – The Simpsons: Tapped Out