In 2015, it was announced that Colfer was working on an adult graphic novel called Illegal with Andrew Donkin and Giovanni Rigano, the team behind the Artemis Fowl Graphic Novels.
On 1 September 2015, Variety reported that Kenneth Branagh had been hired to direct Artemis Fowl for Walt Disney Studios Motion Pictures, with Irish playwright Conor McPherson as screenwriter and Judy Hofflund and Branagh as producers. Colfer confirmed this in a video to Artemis Fowl Confidential, and spoke with RTE Radio 1 about meeting Branagh several times to discuss the film prior to the announcement. The film was originally scheduled for release on August 9, 2019, but on May 7, 2019, it was delayed to May 29, 2020. The film was pulled on April 3, 2020 due to the COVID-19 pandemic, and debuted on Disney+ on June 12, 2020 instead of a theatrical release, to overwhelmingly negative reviews.
Colfer wrote an extended monologue for the Wexford Arts Centre in 2015, before expanding the piece into a full play. My Real Life was originally performed across Ireland and the UK throughout 2016 and 2017, before coming to the Edinburgh Festival Fringe in 2017. It starred Don Wycherley as Noel O'Brien, dictating his final thoughts and regrets to a friend on an old cassette tape. The play received 4- and 5-star reviews from a variety of publications.