Along with his wife Nicole Stinn, he founded Nitrogen Studios Canada, Inc. in 2003, through which he introduced CGI to the Thomas the Tank Engine & Friends franchise.
In addition to working on several episodes of Rugrats and The Wild Thornberrys, he made sequence contributions to the feature film spin-offs of both programs.
Through this new venture, he helped develop God of War (2005), the first entry in the renowned PlayStation game series and the animated film Happily N'Ever After (2006).
Tiernan gained new acclaim following the 2016 release of his feature film debut, Sausage Party (in which he also voiced a potato and a noodle soup can).
Several days after the release of Sausage Party, allegations of poor treatment of Nitrogen Studios employees surfaced in the comments section of an interview with Tiernan and co-director Conrad Vernon, featured on the website Cartoon Brew.