The pop singer played caddy for the world number one during the Masters Par 3 contest in Augusta, Georgia on Wednesday (08Apr15), carrying MCIlroy's bag and clubs as he worked through the famous course.
The pair was walking along a sloped embankment to chat with a Sky Sports presenter and as they came to a stop, Horan slipped over and the weight of the heavy golf bag pulled him down to the ground. The TV footage showed the star struggling to get to his feet before brushing himself off to begin the interview.
Horan, an avid golfer, joked on Twitter.com, "If your (sic) walking around Augusta, don't step on the pine leaves, slippy as hell Hahahaha! #redfaced!"
He suffered further embarrassment when he was given the opportunity to play a shot himself but shanked the ball straight into a nearby lake.
He added on Twitter, "Well that was an incredible experience, thanks @MCIlroyRory for letting me do that, shame I pulled my shot into the water!... Also, can't say I've ever been so nervous in my life than when Rory handed me that golf club, horrendous shot!"
Horan also hit back at outspoken newsman Piers Morgan, who criticised the singer's sporting skills on the micro-blogging site, writing, "When was the last time you caddied at the Par 3 at the Masters Piers? Oh yeh never (sic)! Back to bed old man!"