Katy stood in for Dannii Minogue when auditions hit Ireland two years ago, and ultimately decided to put Niall through to bootcamp after the other three judges disagreed.
The rest, as they say, is history.
Katy, who said in Niall's original audition she saw a "seed of talent" in him, hasn't forgotten, writing to the teenager on Twitter this week: “Congratulations, you didn’t let me down! xo.”
He replied: “Let’s hope it holds out this week! Thank you for putting me thru! we’ve been working hard, I won’t let you down.”
The group are currently number one on the iTunes album chart, with the boys set to become the first ever UK band to debut at number one on the US album charts next week.