"We had discussed the strategy of playing 18 in the practice round on the Tuesday, which was if you're one behind, level or one ahead, you hit driver and try to win. When we got there he was one ahead. There was no question, there wasn't even a thought, it was just, ok, this is it.
"We knew what we had decided, went with driver. For me, he'd played so well, he'd driven it so well for the weekend and particularly on the Sunday. I couldn't see him doing anything else but hitting a great tee shot down the last."
Instead, Harrington's drive leaked right and into the Barry Burn. What followed thereafter presented an immense challenge to both player and caddie.
After taking a penalty drop, Harrington proceeded to pull his third shot left and his ball found a different part of the Burn, the river that meanders through Carnoustie on its way to the North Sea. It was hard for anyone not to think back to the events of 1999 and Flood knew he needed to act fast.
"Initially, when he hit his third shot, I didn't know whether it was out of bounds or not," said Flood, "because it had gone left quickly and he asked me, 'where's that gone?'
"It was the walking referee who told us it was in the Burn. And he just went quiet and, yes, I was thinking about Van de Velde, I'm sure he was thinking about Van de Velde, but over the years we've had different times when things have gone wrong, so I just started talking to him just the same as we would normally, just telling him that the hole wasn't over, a lot of cliches about we'll just wait, get through this hole and see what happens.
"I just kept talking to him and looking for him to give me a response, just say anything. And I just kept saying, 'come on, you hear me,' whatever, and eventually he was like, 'yes, ok, yes'. He wasn't answering me for probably two thirds of the walk down and then he started talking and getting into it.
“He wasn't answering me for probably two thirds of the walk down” RONAN FLOod
"He started answering me back more, talking a little bit before we got to the ball, so at least it was kind of somewhat out of his head. By the time we got down there, he'd worked out the yardage, he'd dropped his ball and he was back into playing golf, so he was fine at that stage."
Under immense pressure, Harrington produced a superb pitch and holed the resulting putt to rescue a double-bogey six. Although that left him one behind Garcia, the Spaniard then finished with a bogey to ensure a play-off was needed.
In a recent Twitter video, Harrington described his pitch on 18 as one of the two greatest shots he has played in his career, together with the stunning five-wood into the 71st hole that effectively secured a successful defence of The Open at Royal Birkdale the following year.
It is clear the three-time major-winner places huge value in Flood's contribution to his Carnoustie triumph.
"I'd hit two of the worst shots of my life," Harrington explained. "I was feeling terrible. My caddie literally had to talk me around. It was 150 yards of a conversation that I didn't want to have and he just kept at me.
"I've never felt that bad on the golf course, I felt embarrassed. My caddie kept at me, kept telling me to play it out, one shot at a time, see what happens. By the time I hit my chip shot from 48 yards as my fifth shot on that hole, I hit it like a teenager showing off. I was back in the zone."
When Harrington and Flood returned to the 18th in the play-off, a two-shot lead over Garcia had been established. There was no chance of the driver making another appearance.
"I wouldn't have given him the driver and he wouldn't have asked for it because we had agreed," Flood added.