The 24-year-old vocal powerhouse gave one of the strongest performances on Tuesday night's show, belting out the rock gospel song "Superstar" from Andrew Lloyd Webber's musical "Jesus Christ Superstar."
Her performance won praise from the judges, but not enough viewer votes for her to return next week. The audience cast more votes for Brooke White, who had to start over after forgetting the lyrics to her song, and Jason Castro, who barely squeaked through his number.
Even though he had been harsh on Smithson in previous weeks, Cowell tried to reassure her by saying she could leave the show with her head held high.
Smithson has already had her share of. After early success as a child actress and model in Ireland, she moved to the U.S. in search of a record deal. Smithson signed with MCA Records, but the label dropped her after her 2001 debut album, "Ultimate High," tanked.
She managed to stay upbeat after learning she'd been given the boot, saying she was very proud of herself and happy. "I'm not upset that I'm going home," Smithson added. "I had the best time here."
Syesha Mercado -- who also had a great night Tuesday with her lively performance of "One Rock And Roll Too Many" from "Starlight Express" -- joined Smithson in the bottom two. The two contestants, who roomed together and became friends while on the show, each had a chance to reprise their songs Wednesday.
Earlier in the show, President Bush and first lady Laura Bush announced that the show's "Idol Gives Back" special had raised more than $65 million for various children's charities.
The evening also included a performance by Cowell's latest find, singer Leona Lewis.
Smithson's exit leaves five contestants to compete on the show next week.
By Judy Rosen