In 2015, two years after her last acting role, IU starred alongside Kim Soo-hyun, Cha Tae-hyun, and Gong Hyo-jin in The Producers, which was named as one of the most anticipated TV series of the year in South Korea. She played Cindy, a diva-like singer who decides to participate in a variety show to improve her public image. Analysing her character's perceived icy demeanor, IU commented that she could relate to her character's "biting attitude" and how similar her character was to herself. The series and IU's portrayal of Cindy were met with lukewarm reviews at the premier but both the viewership and critical response improved over the course of the series, peaking at 17.7% viewership for the last episode. She performed two songs in character, "Twenty Three" and "Heart" (마음), both of which were later included as bonus tracks on her 2015 album, Chat-Shire. While she wrote the lyrics for both songs, she also composed the music for "Heart", which was released as a digital single on May 18. "Heart" topped the charts upon its release and went on to become the tenth best-selling digital single of 2015 in South Korea. "Heart" was among a slew of K-pop songs used by South Korea's Defense Ministry in its propaganda broadcasts to North Korea during the two countries' tense standoff in August 2015, with the aim of advertising the "South Korean youth culture". Through The Producers, IU's popularity in China soared—reportedly receiving offers for several projects from Chinese companies.

After the completion of The Producers, IU participated in the biennial music festival event hosted by the variety show, Infinite Challenge, in which each participating singer-songwriter is partnered with one of the six hosts to create a song for the festival. The preparation process and festival event were aired over the course of seven episodes. She was partnered with Park Myeong-su, with whom she recorded and performed the song, "Leon" (as inspired by the film Léon: The Professional). The live performance event on August 13, 2015, attracted an estimated crowd of 40,000 at Alpensia Ski Jumping Stadium. Subsequent to the final episode's broadcast, "Leon" was released along with the other songs from the festival as a digital track and on the official festival album. "Leon" reached number one on all of South Korea's music charts upon release.

IU's seventh Korean extended play, Chat-Shire, was released digitally on October 23, 2015, prior to its physical release on October 27. IU asserted greater creative control on the album, writing the lyrics for all seven tracks (as well as the physical album's two bonus tracks) and composing the music for five tracks either individually or collaboratively. She was also credited as the album's executive producer. The lead single, "Twenty-Three", topped the national music charts upon release with several of the album's tracks also ranking in the top ten. The album reached a peak position of number four on Billboard's World Albums chart. Billboard described the album as a "must-hear" release, containing "some of [IU's] most personal music yet", and named "The Shower" (푸르던) as its stand-out track.

Despite positive critical reviews and success in the music charts, the album became controversial due to the lyrics of "Zezé" and the audio samples used in the bonus track "Twenty Three" (not to be confused with the similarly titled lead single). On November 4, the Korean publisher of the novel My Sweet Orange Tree, from which IU drew inspiration for the song "Zezé", stirred debate in the entertainment industry on freedom of interpretation when it leveled criticisms at IU for her interpretation of the five-year-old protagonist as a "sexual object". Two days later, IU issued a public statement: "I did not mean to turn Zeze into a sexual object... but I realize that my lyrics have offended many, and for this I apologize." On November 10, the publisher released an apology for its failure to acknowledge "diversity of interpretation." For the bonus track "Twenty Three", voice samples from Britney Spears' "Gimme More" were allegedly used without permission. Billboard ranked Chat-Shire the sixth best K-pop album of 2015, explaining that "IU knows how to take the sounds of yesterday and update them to stunning results.

Prior to the release of Chat-Shire, LOEN announced that IU would not be partaking in broadcast promotions for the album and would instead be holding a national tour from November to December 2015. The Chat-Shire tour began in Seoul on November 21 and continued to Busan, Daegu and Gwangju, before returning to Seoul, where it ended on December 31. In the midst of her concert tour for Chat-Shire in South Korea, IU also increased promotional activities in Hong Kong, China and Taiwan, hosting fan-meetings and concerts in various cities as well as releasing a compilation album, Smash Hits, to the Taiwanese market on December 11, 2015. Containing 16 previously released songs, Smash Hits was published by Warner Music Taiwan and topped the K-pop album chart of Taiwan's leading online music store, KKBOX, on its first week of release. Tickets to her concert of January 10, 2016, in Taiwan also sold out within two minutes of going on sale. Towards the end of 2015, IU hosted SBS's annual end-of-year music program, Gayo Daejeon, with Shin Dong-yup. The program's producers said of their host selection, "IU has been deeply acknowledged for her musical talents, moving beyond the image of 'Korea's cute younger sister'." For her accomplishments in 2015, Ize listed IU as one of the "Persons of the Year", while GQ Korea named her as the "Woman of the Year". In September 2016, IU starred as the lead role, Hae Soo, in Moon Lovers: Scarlet Heart Ryeo, a Korean adaptation of the Chinese novel Bu Bu Jing Xin.