The London-born athlete -- whose parents are both Irish -- revealed how qualifying on Tuesday night left him overwhelmed with shock.
"I was just jumping around the room crying my eyes out. I was just so chuffed. It really is a dream come true," he said.
"To think years and years ago I was told 'you're never going to be able to walk again let alone do gymnastics'....to find out I'm going to the Olympic Games is something that dreams are made of," he added.
Kieran has defied all odds having seen his budding career struck by several devastating setbacks over the years.
Doctors discovered a non-cancerous tumour on his left leg when he was just 10-years old, which eventually left him wheelchair bound. "I had the tumour removed but the operation went wrong," Kieran explained. "It damaged all the nerves in my leg. So I was wheelchair bound for a very long time and told I wouldn't walk again, let alone do gymnastics."
And just two years later, Kieran suffered a head injury when working with a high bar, which resulted in him sustaining brain damage.
"I remember some of the hard days when my mum would wheel me to the kitchen window. I'd just sit out there and watch the cars go past and get my eyes adjusted to looking at things again because my brain couldn't function with things moving quickly and me moving my eyes too fast. That took a long time. I was out of gymnastics for three years in total.
Kieran has hailed his place in the Olympic games as a "dream come true".
"To come from all that and everything that's happened with injuries, I've got the luck of the Irish on my side, most definitely," he joked.
"I wouldn't be here now if it wasn't for my friends and family and coaches and everyone who has stuck around and been there supporting me."
Behan, who will compete in today's floor final at the test event, is already focusing on what he needs to do ahead of the Games.
"I'm going to try and max out my speciality piece which is my floor. I've definitely got the execution, I just need a little bit more on my start value now.
"It's going to be fantastic to represent Ireland."