Kiernan also discussed his experience working on the new series alongside Woody Harrelson.
She is best known for her appearances as Sally Draper in AMC's drama series Mad Men (2007–2015).
Shipka made her broadcast debut on the hospital drama ER at the age of five months.
She also began modelling for commercials when she was a child.
She appeared as a recurring guest star on the show for the first three seasons before becoming a series regular in season four.
Kiernan emphasised their friendship, adding, "He's like a brother to me."
In Episode 12, the Grammy-nominated musician will mentor the top 26 American Idol contestants.