Shirai has been playing video games since he was a kid, and distinctly remembers playing Super Mario Bros. while in the 4th grade. His goal in the video games he works on is to create a stage that causes the player to want to play it over and over again as he did with Super Mario Bros. way back when.
He also likes to create a stage in which the goal is clear, though being able to finish the level won't be so easy. He also wishes to create a perfect balance so that while the game will be hard, it won't be too hard that the player will get annoyed and quit. He played the role of director (along with two others) on Mario Kart: Double Dash‼ and assistant director on Donkey Kong Jungle Beat. He also designed and fine-tuned many of the planets and levels featured in Super Mario Galaxy. He took the lead on level design on further 3D Mario titles.