[2] As a sophomore baseball player in 2014, he threw 153 pitches in the NYSPHSAA Section 1 Class A championship game to beat Lakeland High School.
[4] Although expected to be an early pick in the 2016 Major League Baseball draft, he fell to the New York Mets in the 32nd round because of signability concerns.
[10] As a freshman at Elon in 2017, Kirby appeared in 16 games (five starts) and had a 1–3 record with a 4.84 ERA, striking out 55 batters in 61+ 1⁄3 innings.
[14] That summer, Kirby played in the Cape Cod Baseball League for the Harwich Mariners, where he posted a 1.38 ERA in 13 innings.
[32] He made his MLB debut that day, starting against the Tampa Bay Rays, throwing six scoreless innings and striking out seven batters.
[37] Kirby made his MLB postseason debut on October 8 in the Mariners 10–9 win over the Toronto Blue Jays in the second and final Wild Card Series game.
He issued a one-out walk to Matt Chapman, then retired the next two hitters to finish the game and send the Mariners to the American League Division Series (ALDS).
[43] He pitched in front of his home fans in Seattle, picking up a blown save after allowing a run in the fourth inning.
[45] After a loss to Tampa Bay on September 8, Kirby criticized manager Scott Servais for leaving him in the game past the 6th inning.
Kirby said after the game, "I always loved watching that guy pitch, even though he's a Red Sox player and I'm a born Yankee fan."
The single Knuckleball induced a swing and miss from that years American League MVP Runner-up Corey Seager.
Seattle catcher Cal Raleigh, who caught Kirby's knuckler, grew up a Red Sox fan.
[52] Kirby struck out a career-best 12 batters on April 23, pitching seven scoreless innings in a win over the Arizona Diamondbacks.
[54] Kirby suffered one of the worst starts of his career on August 13, allowing 11 runs, only six of them earned, while recording 11 outs in a blowout loss to the Detroit Tigers.
As a child, he was a Yankees fan, with Jorge Posada, Derek Jeter, and Bernie Williams his favorite players on the team.