The comedian, 52, stars in the bittersweet comedy drama, the title of which is taken from the Northern Irish town in which it is set, alongside Irish actress Seana Kerslake.
It comes as Kielty took over as host of RTE's The Late Late Show on Friday, attracting an average audience of 830,000 on RTE One and RTE One +1, according to the broadcaster.
Speaking at the London premiere of the film in The Mayfair Hotel, he told the PA news agency that taking on the role was "left field" for him.
"I've been doing stand-up, the only acting I'd ever done in my life was a play which was in Belfast that transferred into the West End," he said.
"When the guys came to me with a script, I genuinely thought that they wanted me to give them advice on one of the character roles because I was a comedian in Northern Ireland and there's sort of an element of that in the movie.
"So whenever they offered me the part, this was not planned, none of this was planned.
"This is the greatest surprise and the most unexpected joy I've had in my career."
Kielty said his character is "really not funny at all".
He added that his co-star Kerslake "steals every scene in terms of comedy".
Kielty continued: "At the heart of the movie is a connection between people that maybe just pass each other in life along the way and how they can maybe fix each other and put themselves together.
"So there's a lot of heart in the movie as well."
Kielty said it had been a "crazy" few days with the show and film launch, adding: "At one point I was hoping to go on The Late Late Show as a guest to promote this movie.
"So now I'm hosting The Late Late Show and we've got this out on the one weekend.
"Which is just crazy stuff."
The presenter said the reception to his first Late Late Show hosting appearance has left him "mind blown".
He added at the event: "Like absolutely blown away by all of the love going into it and the messages on the night and the reaction.
"We were really lucky that we got a big audience.
"Lots of people were watching it and just still buzzing off that."
Ballywalter is in cinemas from September 22.