The former One Direction singer (23) and ex MTV presenter (31) flew the flag for the Irish at the British Summer Time Festival in London's Hyde Park on Sunday to catch a performance by The Lumineers.
The pair were treated to VIP seats at the popular event where the pals partied together, documenting their antics on Instagram. Despite the age gap, Whitmore and Horan have been good friends for years and she said she thinks of him more as a brother.
"I'd rather you actually asked me about somebody else other than Niall, to be honest. I was never seeing Niall. He's a child," she told heat magazine in 2014.
"The most embarrassing thing about that is that people thought we were together. I love Niall, he's lovely but I'm 28. He's a boy and he kinda looks like my little brother, so no."
Whitmore was recently linked to Game of Thrones actor Richard Madden, who she was reportedly dating for six months before splitting last August, while Horan said he's single, adding he's too "flat out" for a relationship.
"Ive just been flat out, living in the studio for what feels like forever," he said. "Maybe when I find time, but I'm in no rush! I'm 23, man."