Larry Bass told the Herald that both TV beauties were in the running for the job, but Carlow girl Kathryn edged it by a nose.
He said the eventual decision to chose Kathryn (32) over Grainne (37) was made "on a whole range of things".
"We looked at Grainne, we looked at Brian (Ormond), we looked at lots of people. We had to make a decision which was thought was best.
"If you listened to everything being speculated about people, you wouldn't be able to make a decision or a programme -- both of those people could have done the show. It's a hair's breadth as to why we move from one person to another.
"One thing's for sure -- Kathryn can deliver this, absolutely in spades. She has an empathy, she's very popular around the country and I think it's time for her to get the big show. And this is a huge show... this is going to be a juggernaut for the schedule."
He also took a pop at shows like The X Factor, where it's all about "drawing out the tears" with the sob stories. "The stories will always come out and we're a great story-telling nation but we're not setting out, as we did in You're A Star or Popstars, to find them out at the start. We're really starting with a blank sheet and the person's voice," he continued.
The series starts with a blind audition, where the four judges can't see the competitors, and Larry he said he's confident of finding "another Adele or Damien Rice".
Meanwhile, Kathryn has described the gig as her "dream job" and said she can't wait for the new challenge, alongside TG4 newcomer Eoghan McDermott as co-host.
However, she admitted that reports about it being handbags at dawn between her and Grainne did become 'jaded' after a while as their male counterparts didn't get the same treatment.
"It's not rivalry, it was competition and there were seven of us up for this job and everybody wanted it," she said.