The boyband formed on the reality Tv show in 2010, and went on to huge international success after finishing third.
Horan initially tried out for a place on the show during open auditions at the Croke Park stadium in Dublin, Ireland, and he celebrated his triumphant return to the venue as the band performed to a packed crowd there on Friday night.
Speaking before the show, he told Britain's The Sun newspaper, "I travelled to Croke Park on the bus to sing for The X Factor and to think I'm back with the band headlining for three nights... It's hard to take in. This is the one I've been waiting for."
In a series of posts on Twitter.com after the show, he writes, "Ireland! thank you so much for tonight! That was the best experience of my life! Incredible night! Loved every second of it! Love you!..."
The band plays two more nights at the stadium on Saturday (24May14) and Sunday (25May14).