"I'm regarded as a cold so-and-so," he said. "Very impersonal. I think it's part of being of benefit to an athlete that you have to be a little bit..."
"Hard-necked," volunteered the softly spoken Irishwoman sitting beside him, with a little smile. The understanding and mutual respect of this coach and athlete are central to the achievements of the last year, during which McKiernan has made the most dramatic arrival on the women's marathon scene since Liz McColgan.
Last September, the girl from Cornafaen surpassed the girl from Dundee as the fastest marathon debutante, winning the Berlin race in 2hr 23min 44sec. Seven months later she secured the biggest prize of her career by winning the Flora London marathon, a title she will defend on 18 April next year, and last month she won the Amsterdam Marathon in a best time of 2:22:23, missing the all-time best set by Ingrid Kristiansen of Denmark by just over a minute.
Since she became the first female from County Cavan to win the Ulster Championships, McKiernan has progressed inexorably through the ranks - and up the distances. She now arrives at the event which she and her coach have always believed was her natural one.
"Ten years ago we had physiological tests done at Trinity College, Dublin, which showed she was best suited to the marathon," said Doonan, a primary school principal from Carrigallen.
McKiernan's joshing of the man who set her on the path to first fame and then riches - she picked up $55,000 (pounds 35,000) for winning the London title plus a time bonus of $10,000 (pounds 6,250) and a healthy appearance fee - is almost daughterly. She is, as it happens, only a couple of years older than the eldest of Doonan's seven children.
He in turn speaks of her in protective, paternal terms. "I would never want to push her so hard in training that it harmed her," he said. "She should have five or six good years ahead of her unless she decides to give it all up and get married. And if she does, good for her. I'll speak at the wedding."
Doonan, however, finds that he does not have to force the pace too often, as McKiernan is - and has always been - someone who shares his dedication. "It was there right from the start," he said. "There's a little bit of a similarity between us, all right. When the target goes up she's very, very, very single-minded.
"Over the years, that has meant making sacrifices. You can't afford to go out drinking and socialising if you want to make a serious go of running. "She is one hell of a competitor. She loves winning."
For all that, McKiernan had to get used to more than her fair share of narrow defeats in the years before she arrived at the 26-mile distance - notably in the world cross country championships, where, excruciatingly, she was runner-up consecutively between 1992 and 1995.
The grace with which she accepted those results - and the vocal and colourful support supplied by her clan of supporters - were widely celebrated. Every year she pointed out the incontrovertible fact that silver medal in the world cross country championships was a fine achievement.
Winning the 1994 European title did something to redress the balance, but it was only after she had concluded her main track career by reaching the 1996 Olympic 10,000m final, where she was 11th, that she began to prepare for the event which has allowed her to achieve clear victories.
Doonan, particularly at times of disappointment, has regaled McKiernan with one of his favourite sayings: "It's better to deserve honours and not to have them, than to have them and not deserve them." Now she finds herself in the happy position of both having and deserving the honours.
Defending the title against a field that will include the 1996 winner McColgan and 1997 winner Joyce Chepchumba of Kenya will be far from easy for McKiernan, who noted the presence of Portugal's Olympic 10,000m champion Fernanda Ribeiro, who will make her debut.
"I ran very fast in my first marathon," McKiernan said. "She could do the same."
In an effort to standardise the marathon to the point where official records - rather than bests - can be set, the London event is introducing a $125,000 (pounds 78,125) bonus for anyone beating the world record for a women- only race.
It currently stands at an unratified 2:21.46, set by Japan's Naoko Takahashi at the Asian Games this month.