The visitors trailed 19-10 at halftime, and Kirwan says they need to mature as footballers.
"It's a bit of a reality check for us, we know how we can play.
"We've got the ability, although the side's very, very young, but I just think they just need to learn about preparation.''
The Cheetahs win secures them a playoff spot for the first time.
Meanwhile the Highlanders have edged the Hurricanes 49-44 for just the third win of their season, while the Stormers have beaten the Kings 24-12.
The Bulls have scored a late try to narrowly beat the Sharks 20-19, and move ahead of the Chiefs atop the standings.