Flack won his age category at the tough 70.3 Dun Laoghaire event, which attracted a top class international field.
He was first male in the 25-29 age group, finishing in a time of four hours, 45 minutes and two seconds.
The race included a host of international professional triathletes and Flack was competing with them all the way and finished 14th overall and was fourth British athlete home.
Great Britain’s Elliot Smales took overall victory, with America’s Andy Potts second and Ireland’s Bryan McCrystal third. British star Emma Pallant, who finished a couple of places behind Flack, won the women’s title.
Windy conditions and the tides made the swim challenging for all the competitors. Former GB international modern pentathlete Flack completed the 1.9km swim in 31.06 minutes. He then clocked 2:48.51 for the 90.1km cycle, which included some tough climbs. The 28-year-old finished the competition off with an impressive 1:19.23 for the half marathon run.
By winning his category Flack has secured at place at the 2019 World 70.3 Championships which will be held in Nice, France.
Flack’s next major competition will be the ETU Middle Distance European Championships in Ibiza on October 28.