The Pirates prospect allowed a pair of hits over six scoreless innings in his Double-A debut Sunday as the Altoona Curve shut down the Harrisburg Senators, 4-1.
"It felt great and it was nice to get out there," Irwin said. "I just tried to take it as just like any baseball game, but I was a little nervous and I had some jitters.
"My fastball was all over the place in the first inning and I think I threw 20-something pitches, but after that I was able to settle down."
After hitting the second batter he faced, the 24-year-old right-hander retired the next 13 Senators before Alex Valdez broke up his no-hit bid with a two-out infield single in the fifth.
"I'm usually pretty aware of what's going on out there, but I was just trying to pitch to contact, get some ground balls, eat some innings and get quick outs," said Irwin, the Pirates' 21st-round pick in the 2009 Draft.
Pitcher Erik Arnesen singled leading off the sixth and Chris Rahl drew a two-out walk, but Irwin got Tyler Moore to fly out to center field on his 79th and final pitch of the afternoon.
"It was definitely nice," Irwin said of his Eastern League debut. "My pitching coach came out to give me a quick breather before [Moore's] at-bat and he told me to bear down and get the guy out. After that first inning, where I threw all those pitches, I didn't think I would even get that deep."
Irwin went 5-0 with 40 strikeouts over 53 1/3 innings with Class A Advanced Bradenton before learning he would be promoted Thursday. His 2.03 ERA with the Marauders would have ranked second in the Florida State League if he'd pitched enough innings to qualify.
"It's a funny story because I had been feeling sick and I was taking a couple days off and I thought I was going to need a doctor's appointment. Then the club called and said I was going to Double-A," Irwin said. "It was the best news I could have heard to feel better."
Last year, his first full season in the Minors, Irwin fanned 111 batters over 113 innings and went 6-3 with a 3.35 ERA in 23 games for Class A West Virginia. The Ole Miss product made his pro debut in 2009 with short-season State College .
Pitching at his fourth level in three years, Irwin hopes he can continue to impress.
"I knew Double-A was going to be harder, but I kept my normal routine," the Tennessee native said. "The biggest difference was their plate discipline because they didn't chase as much. I threw some pretty good curveballs, and it didn't even look like they were going to offer.
"Now I need to stay with what I am doing well. I need to throw my sinker down in the zone and my curveball late in the count."
Former first-round pick Bryan Morris allowed an unearned run on two hits while striking out four over the final three innings to record his third save in as many chances.
Quincy Latimore led the Altoona attack with a solo homer, double and three runs scored.
Arnesen (3-1) took the loss after allowing two runs -- one earned -- on three hits and two walks over six frames. He struck out seven.